system
Given the following post, respond as if you're commenting on Reddit:
human
Why is Dark Matter called 'matter'?

Aside from the fact that the word 'dark' is a placeholder term. As far as I understand we have only measured unexplained gravitational effects. Wouldn't it be more accurate to call it 'dark gravity'? Is matter literally the only thing we know of that could produce such effects?
gpt
We really do believe it is most likely an actual form of matter. It is the simplest explanation, and doesn't require arbitrarily changing the fundamental laws of physics to fit our observations.

And that's really the only alternative to dark matter - just inventing new laws for gravity. This is tricky, because General Relativity is really a beautifully minimalist theory. It's the simplest possible result you get from a very small number of assumptions. But if you want to make GR more complicated, there are very few constraints on what you can do - you can modify it however you want. This makes it very hard to prove that these theories actually describe gravity, and aren't just fudging the physical laws to get the right answer.

On the other hand, dark matter being made up of some exotic particle is much easier to constrain and understand. We already know there are particles that are invisible and don't interact electromagnetically - neutrinos, for instance. We also know that we probably don't have a full catalogue of all subatomic particles - there are apparent holes and symmetries, and there are very natural places for possible dark matter particles (basically, fat neutrinos) to live. And, as kinematic particles, we can quite robustly simulate what a distribution of dark matter would do, even if we don't exactly know what it's made of - it turns out it doesn't matter if something is a cloud of black holes, or a cloud of stars, or a cloud of dark matter particles, they all follow the same basic dynamical equations. So we can test things more robustly. Rather than setting up dark matter to fit our observations, we can throw a near-uniform distribution of dark matter into a simulation and see what it produces. And what we get is galaxy-sized blobs of dark matter, with just the right mass profile to give the rotation curves we observe.

One specific "smoking gun" though is the direct evidence of the Bullet Cluster. Here, two galaxy clusters have collided. Galaxy clusters have 99% of their visible mass in gas, and this gas has smushed together where the galaxy clusters first hit each other. However, the stars just flew past each other, as should the dark matter. Using gravitational lensing, we can find out where the mass is - and we find that the mass is not in the gas, where 99% of the visible mass is. Instead, it's around the stars, where the dark matter should be. If you modified gravity instead, you'd still expect to find gravity concentrated on the gas, but that's not what we see.