Ask Ethan: Could there be dark matter aliens out there?

When it comes to the entirety of the Universe, the “stuff” that we’re made out of — atoms and their constituent particles — plays an important role. Protons, neutrons, and electrons, in atomic, nuclear, and ionic configurations, make up all of the light-emitting and light-absorbing structures that we know of. Stars, galaxies, gas, dust, plasma, black holes, and more are all made of these simple ingredients, with little room for anything else. And yet, they make up just 4.9% of the Universe. Overall, the most abundant form of cosmic energy is dark energy, making up 68% of the cosmic energy budget, followed by dark matter, which makes up 27% of all that’s out there.

Well, if the normal stuff, or stuff like us, can make such interesting shapes, structures, and creatures, what about the dark stuff? That’s what our reader Wesley wants to know, writing in to inquire:

“…is it possible we’re the outliers? If dark energy… and dark matter [are so much more abundant than normal matter], could there be this whole other dark world of elements, energy, etc., that evolved dark intelligent life? Are they wondering what we are while we’re wondering what they are? If we go down the rabbit hole a little, could UAP be dark intelligent life probing our “visible” side?”

It might seem like a wild, speculative idea, especially given how little we know about dark energy. But we know a lot more than you might think, and even though this scenario is fascinating, it’s very tightly constrained. Here’s why.

This supercomputer simulation shows the emergence of a rotating disk after hundreds of millions of years of cosmic evolution from gas and dust; the simulation also includes stars and dark matter, which are not shown here. If the dark matter were visible, it would make an enormous halo much larger, in radius, than the entire size of the image shown here.

The key property of normal matter — the stuff that atoms are made of — that enables it to form complex, bound, collapsed structures, is electromagnetism. The fact that the matter we know of and are familiar with, the matter that makes up galaxies, stars, planets, molecules, and all forms of life, can make the structures we’re familiar with is because of two major properties that it possesses.

1. The fact that matter, as far as we know it, comes in two main varieties: heavy, positively charged atomic nuclei, made up of protons and neutrons, and light, negatively charged electrons, which can form bound states (atoms and ions) with those atomic nuclei.
2. And that fact that those two main species of charged particles, nuclei and electrons, create electric and magnetic fields and interact with light, simply by existing with the properties they possess.

This is the reason that normal matter can form bound states, like atoms. It’s the reason that normal matter can absorb light, doing things like becoming ionized or entering excited states. It’s the reason that atoms can link up and form molecules and even more complex structures. And it’s also the fundamental reason behind the fact that normal matter can collapse: because it can radiate energy away, in the form of light.

Six of the eight quadruply lensed systems, from a 2020 study, used to place the best model-independent constraints on dark matter’s temperature/mass from structure formation alone. This method involved no dependence on the interaction between normal matter and dark matter, but rather the variations in the lensed images allowed the researchers to discover the presence and properties of dark matter substructure within these lensed systems. It clumps into diffuse halos with sub-halo substructure inside of it, but yields no evidence for gravitational collapse or for the emission, ever, of dark radiation.

You might wonder, then, about dark matter. Dark matter, very definitively, does not have these properties. There are a lot of things we fundamentally don’t know about dark matter, including:

• whether it’s heavy or light, in nature,
• how many dark matter particles there are,
• whether there’s just one species of dark matter or whether there are many types of dark matter,
• whether there are “dark properties” like dark radiation, interactions that only exist between dark matter particles, or self-interactions that dark matter experiences,
• or whether it’s even a particle at all, or some novel type of material, such as a massive, dark fluid.

The only meaningful information we have about dark matter comes indirectly: from its gravitational effects on the Universe. This means that we can look at signatures for how dark matter bends the fabric of space, gravitationally, and observe how light that’s emitted along our line-of-sight, from behind that dark matter structure, is affected by dark matter’s presence. We’ve been able to determine an enormous amount about dark matter from this, with a few observations standing out as colossally important: strong and weak gravitational lensing, the large-scale structure formation of the Universe, baryon acoustic oscillations, the cosmic microwave background, and the dark matter-tracing distribution of intracluster light.

This Hubble image of massive galaxy cluster MACS J1206 shows the arcing and smeared-out features caused by the gravitational light-bending of the foreground galaxy cluster. Small-scale concentrations of dark matter, represented in blue, have been reconstructed based on the lensing data. Combining this lensing information with intracluster light information, which is another, independent tracer of dark matter, can reveal its presence and distribution as never before. We discover, through analyses like this, that all dark matter haloes consist of a rich set of dark matter substructures.

When we put all of these lines of evidence together, we find that dark matter is incredibly consistent with interacting only gravitationally: with no electromagnetic or nuclear interactions, and with no self-interactions at all, for that matter. Our simulations, especially on large cosmic scales, show us quite definitively that dark matter must have been:

• born cold, or moving slowly compared to the speed of light, even at early cosmic times,
• clumped up into not just giant halo-like structures, but containing a rich set of smaller halos (i.e., dark matter substructure) embedded within them,
• incapable of shedding even a substantial fraction of its linear and angular momentum, as it remains diffuse and hasn’t collapsed in any measurable way, even after 13.8 billion years,
• and yet, must be massive, even though it exhibits no electromagnetic or nuclear interactions.

Perhaps the most striking property of dark matter is this, though: the fact that, despite incredibly sensitive searches, we’ve never been able to detect it directly. We’ve never seen it absorb or emit light; we’ve never seen it absorb or be emitted by any quantum of known (normal) matter; we’ve never witnessed it recoil, or strike and transfer momentum to, a particle of normal matter; we’ve never created a signature in the laboratory that’s consistent with a dark matter particle. For all our searching, direct detection efforts have come up empty.

These two plots show the spin-independent cross-section constraints between WIMPs and dark matter from XENON (left) and for XENON compared to PANDA-X and LZ, which are two competing contemporary dark matter experiments. With its current improvements, XENON will be competitive with both and may yet pass them all over the next few years.

You might think, then, that would rule out the possibility of dark matter binding together to form a bound structure of any type. There should be no dark atoms, dark planets, dark stars, or dark beings out there; that’s the straightforward expectation.

But then you might think to ask: are there any indirect observations, astrophysically, that hint that dark matter could actually, at some level, interact with itself, alone, even if it doesn’t interact with normal matter or with light or with any of the known Standard Model particles?

And there is a place we can look for that: to the smallest-scale structures that are gravitationally bound, that contain points of light within it (i.e., stars), and that also should still have copious amounts of dark matter within them. If you look to individual galaxies, you’ll find that for large, Milky Way-like galaxies, they’re very consistent with that same dark matter ratio that appears to persist all throughout the cosmos on large-scales: that dark matter “outmasses” normal matter by about a 5:1 ratio. As we go to smaller scales, however, we start to see that ratio change. On average, it increases the smaller the mass of our galaxy in question is, with ratios looking more like 10:1, 100:1, or in the most extreme cases, 1000:1 or more.

Many nearby galaxies, including all the galaxies of the local group (mostly clustered at the extreme left), display a relationship between their mass and velocity dispersion that indicates the presence of dark matter. NGC 1052-DF2 is the first known galaxy that appears to be made of normal matter alone, and was later joined by DF4 in 2019. Galaxies like Segue 1 and Segue 3, however, are particularly dark matter-rich; there are a wide diversity of properties, and the dark matter-free galaxies are only poorly understood.

What could be going on here? Could dark matter be interacting in some way that goes beyond what we presently think?

It’s possible, but the simplest explanation doesn’t require it. Instead, while normal matter and dark matter are both expected to gravitate, normal matter can do things dark matter can’t, including:

• gravitationally collapse,
• radiate heat away (i.e., cool),
• form stellar structures (i.e., stars),
• undergo nuclear fusion,
• emit radiation,
• and be pushed outward by that electromagnetic radiation.

Just as star-formation creates winds and outward pressure, it’s normal matter, and normal matter alone, that gets blown outward by these processes involving particles of the Standard Model. If there’s a large enough gravitational potential well — caused by normal and dark matter combined — that houses these forming stars, then they won’t escape the galaxy they’re in, and that dark matter-to-normal matter ratio will remain constant. But if there’s too little overall matter, the dark matter will persist, untouched, while only the normal matter gets expelled, increasing the dark matter-to-normal matter ratio at late times.

However, the dark matter will be affected by the redistribution of this normal matter: from the central regions as well as farther away, toward the outermore ones.

When a massive particle passes by a large number of other particles that it only experiences gravitational interactions with, it can experience dynamical friction, where the moving particle will slow down as a consequence of its gravitational interactions with the particles in the medium it passes through. Relative velocities are the quantitative key.

Dark matter, remember, as far as we can tell, behaves like a species of massive particle that only interacts gravitationally. It turns out that means the following.

• As normal matter forms stars and those stars emit winds and radiation,
• those winds and radiation push the normal matter outward,
• expelling some of it from low-mass galaxies and removing it from the central regions of galaxies in all cases,
• and that motion of normal matter changes the gravitational distribution of matter, overall, within the galactic center,
• which causes the dark matter’s distribution to change as well within the galaxy, particularly in the central regions.

We give a counterintuitive name to this phenomenon: dynamical dark matter heating. Unlike conventional heating, which involves the transfer of either kinetic (energy of motion) energy or thermal (heat) energy, via collisions or radiation, dynamical heating is an effect where it’s the motion of particles, and their gravitational influence, that changes the motion of other particles. This dynamical dark matter heating, due to feedback effects from the motion of normal matter as a consequence of star-formation, can play a major role in most of the smaller galaxies: those with about 100,000 stars inside or more.

But that means there’s some wiggle room in the most extreme cases: for the galaxies with the smallest stellar masses of all.

This “almost dark” galaxy, nicknamed Nube, is an incredibly diffuse galaxy found within a grouping of many other galaxies. It is thought that this ultra-diffuse galaxy, which has only a small smattering of stars inside a large mass of neutral hydrogen, owes its properties due to environmental factors. With so much hydrogen and so few stars, it represents a fascinating outlier among conventionally known galaxies. Its past star-formation history has been largely erased over the billions of years that have passed since its most recent major star-formation episode.

These ultra-low-mass galaxies, some of which are very puffy and/or very diffuse, are the one location in the Universe that is most suggestive of a possible self-interaction for dark matter. If dark matter:

• is collisional and has a very large cross-section with itself,
• obeys some sort of exclusion rule the same way fermions obey the Pauli exclusion principle,
• or can stick together, even a little bit, to form some type of bound state with itself,

this could help alleviate the observed tensions between the properties of these galaxies and the standard theory of collisionless, cold dark matter. That’s where the greatest amount of wiggle-room for self-interacting dark matter lies.

But, going back to the original question, could this admit “dark, intelligent life” of some type?

It’s really, really unlikely. Life, as far as we know it, requires a metabolism and the ability to reproduce itself, at minimum. Intelligent life requires much more: the ability to harness energy, to use tools and develop technology, and — if we want to go a step further and hope this is a spacefaring civilization made out of dark sector “stuff” — to journey from one location to another under their own power. Dark matter, based on what’s known about it, even if it possesses some very weak form of self-interaction, shouldn’t be able to do any of this.

By mapping out the three-dimensional positions of galaxies over a large volume of the Universe, scientists within the DESI collaboration have uncovered some (but not overwhelming) suggestive evidence that the strength of dark energy has weakened (and is weakening) over time. Dark energy, however, cannot be clumpy, cannot radiate, and is not inhomogeneous in space; we cannot form any bound structures out of it for certain.

What about dark energy, then?

The situation is even worse. For dark matter, at least it has the ability to clump up together, owing to its non-zero rest mass. (Or, more precisely, its behavior that is consistent with having a non-zero rest mass.) But for dark energy, it has no rest mass at all, and behaves as though it’s a form of energy inherent to empty space itself, alone. It doesn’t clump, it doesn’t possess inhomogeneities, it isn’t more (or less) clustered wherever normal and dark matter are present. Instead, it behaves as though it’s perfectly uniform, in all directions and locations, and does not appear to interact in any other way, even if it turns out (and the evidence is very dubious, here) to vary with time.

Most stringently, we can be 100% confident that unidentified aerial phenomena — currently known as UAPs and formerly known as UFOs — can absolutely not be made out of either dark matter or dark energy for one simple reason: they’ve been directly observed! We have very tight experimental and observational constraints on how much dark matter and dark energy can interact with normal matter and with light, and the already-observed UAPs/UFOs are clearly emitting light in a way that rules out any plausible dark matter/dark energy explanation for them. Whatever they are, they can’t be made of the “dark stuff.”

In 2009, this odd, spiral-shaped light show appeared over Norway, frightening many residents and leading to a large number of claims of a UFO sighting. The culprit turned out to be a Russian military missile, which failed upon launch. Whether any UFOs/UAPs are extraterrestrial in origin, or come from intelligent alien activity, has yet to be established.

And that’s the full story. Any intelligent aliens that are out there, flying through the galaxy or traversing the Universe, must have some way of propelling themselves, which requires some sort of energy-emitting reaction. Because we can’t detect that energy in any known form — such as the forms of light or normal matter — the only other option is that whatever energy is emitted is completely dark as well. Dark energy can’t participate in that; it must be dark matter. But we already have very tight restrictions on what dark matter can and can’t do, as we’ve observed enough about the Universe, the matter in it, how it’s gravitationally distributed, and how the dark matter in it doesn’t collapse or decay, that any scenario where dark matter becomes intelligent aliens is highly constrained, if not ruled out entirely.

However, this should further spur us on to learn more about dark matter. That includes probing more deeply into its nature: does it interact at all with normal matter or with light? Does it interact weakly, and can we probe it with neutrinos or nuclear interactions? Can we learn more about it by studying, in greater detail than ever before, the behavior of the lowest-mass galaxies and by tracing the motions of individual stars within them?

We have to face up to the fact that the “normal stuff” in the Universe represents only 5% of the total energy of what’s out there. Dark energy may be smooth and uniform, but dark matter isn’t. If we want to know what is and isn’t possible with it, we’ve absolutely got to learn as much about this mysterious substance as possible. After all, on a cosmic scale, it’s much more important, energy-wise, than anything else we’ve ever discovered is.

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