The Bullet Cluster shows an offset between the stuff that gravity sees (blue) and the stuff that emits light (pink) (Credit: NASA)

WTF, Universe? Invisible matter? Is this a joke to you?

Whether dark matter exists or not, the Universe is surely fucking with us.

Bridget Falck
May 3, 2018 · 6 min read

What now, Universe? What is it this time? It is hard enough to wrap our feeble human minds around the expanding, rippling, non-curvy but possibly once-inflating, larger-than-you-can-imagine Universe, and now you are telling me most of the stuff in it is literally invisible?

Honestly, it seems like a cheap trick, like the writers of a low budget sci-fi movie just gave up.

[MONTAGE: Our hero scientists are slowly and methodically unlocking the secrets of the Universe or whatever.

But wait! Galaxies are rotating super oddly!]

SCIENTISTS: “WTF?”

HERO SCIENTIST 1: “I’ve got it! There’s stuff in galaxies besides stars and gas, but… it’s invisible. And, uh, there’s way more of it than the visible stuff that makes up literally all of chemistry!”

HERO SCIENTIST 2: “OH. MY. GOD! It’s going to pull the entire Universe into one ultra mega black hole, unless we stop it — with science!”

[HERO SCIENTIST 1 AND 2 MAKE OUT]

Who am I kidding? I would totally watch that movie.

But it’s not a sci-fi movie. It’s REAL.

Except for that last bit about the black hole… probably… that’s basically the story of how most astronomers are pretty sure that dark matter exists. And the dark matter skeptics — the ones who think the idea of “invisible matter” is a load of bull — wish that they could roll their eyes as epically as Liz Lemon.

This (7th) post in the “WTF Universe?” series is long overdue, so let’s get right to it!

Dark Matter

The Evidence

Some of the first evidence for dark matter was due to the work of Vera Rubin in the 1960s and 70s, who unfortunately passed away before she could receive the Nobel Prize she deserved because Nobel Prizes are bullshit and sexist as fuck. After creating her own women’s restroom because the Palomar Observatory didn’t have them, Rubin scienced the shit out of some galaxy rotation curves, observing that they were flat. This means that the rotation speed of stars in galaxies remains relatively constant the farther they get from the galaxy center, instead of decreasing as one would expect based on reasonable assumptions, including:

  1. Gravity works the same in galaxies as it does everywhere else in the Universe;
  2. Galaxies don’t contain invisible matter; and
  3. There are no faeries fucking around with our observations as part of a cosmic prank on humankind.
The measured galaxy rotation curve (white) is much larger than expected (red), and it’s probably not because of faeries. (Credit: Queens University)

These days, most physicists and astronomers believe that the second assumption is false, while some argue that the first assumption is false. (Much to my disappointment, no one is arguing seriously for the existence of faeries. Which is just how they like it.) The “invisible matter” that might be flattening galaxy rotation curves is what we call dark matter.

At this point we have to make a distinction between truly invisible and merely hard to see. It’s possible that there are a bunch of tiny black holes or cold, dead stars that we didn’t know were there because they are very hard to detect. But, there would have to be a shit ton of them to flatten out galaxy rotation curves. So, these days when people say dark matter they usually mean truly invisible — not only can we not see it, it is not even possible to see it because it doesn’t interact with light or with normal matter except through gravity.

Besides galaxy rotation curves, there are many other measurements that point to dark matter being this super weird invisible thing. The pattern of the Cosmic Microwave Background radiation — the light that was emitted everywhere in the Universe about 300,000 years after the Big Bang — can be explained better with it than without it, and suggests that most of the matter in the Universe is dark matter! Something like 4/5ths of the matter in the Universe could be dark matter AND WE HAVE NO FUCKING CLUE WHAT IT IS.

That is to say, we have plenty of clues, and plenty of ideas, but they all suck a bit in their own unique way.

Dark Matter Candidates

It is still possible that some or all of the dark matter is in the form of regular matter that is just very hard to see. These are called MACHOs (Massive Astrophysical Compact Halo Objects), like black holes, and RAMBOs (Robust Association of Massive Baryonic Objects), like white and brown dwarf stars. Or, dark matter could be some new type of matter that doesn’t interact with normal baryons, called WIMPs (Weakly Interacting Massive Particles).

PSA: The field of astronomy collectively apologizes for letting some douche-bro invent the acronyms.

WIMPs were the favored candidate for a while and may still be, but particle accelerators have continued to fail to find any evidence for them in the debris of smashed protons; nor have we been able to detect the signal of their annihilation as they fly through space.

Other candidates for dark matter include axions, which are hypothetical particles invented to solve a problem in quantum chromodynamics. Yes, this is a legitimate area of physics and no, it does not involve uncountable tiny rainbows winking into and out of existence. Unfortunately.

There is also a bunch of other weird shit that has been hypothesized as a dark matter candidate, and one of them might end up being right, but also, theorists gotta eat. My personal favorite is sterile neutrinos. Don’t ask me why.

A Venn diagram on crack showing the landscape of dark matter candidates. (Credit: Tim Tait)

Of course, it is always possible that postulating the existence of literally invisible matter is not the right way to go, and instead of needing a new thing to solve the puzzle, what we need is a new theory of gravity. There have been various problems identified with the so-called standard model of cosmology, which includes things like dark matter and dark energy which we definitely do not understand. Most problems come about when we try to compare our predictions from simulations to our observations of galaxies. But it turns out galaxies are pretty complicated too, so it could be that we just don’t understand them well enough. Do we even know anything for sure? I dunno.

None of this will be settled soon. In the long gap between now and posting the previous piece in the #WTFUniverse series, there has been new evidence that challenges the existence of dark matter, a different study that points to it being much weirder than we ever thought, and yet a third that claims it definitely does exist (because one galaxy has none). Maybe someone should seriously look into the faerie hypothesis…

In the next post we will discuss dark energy which, if you can believe it, is on even shakier ground than dark matter. It is so difficult to explain that some scientists are completely comfortable inventing the multiverse in order to make sense of it.

Bridget Falck

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Professional astrophysicist. Amateur philosopher. Human.