[bmitc]

Dark matter is the name for unexplained mass that doesn't interact like typical matter. The existence of such mass is needed because there are galaxies that don't match what is predicted by general relativity. So either general relativity is wrong on the particular predications or there is extra matter.

The universe is expanding. However, not only is it expanding, but the expansion is accelerating. It isn't clear what is driving this, and source of the acceleration was given the name dark energy.

The "dark" in these names are part we don't know what these things are yet and part we can't see them yet with our current observational methods.

> I find it kind of funny that humans are so confident that our models of reality are correct

I don't think this is the case. If you have another model that replaces general relativity, I think people would be very interested in it. People have tried several times to explain the observations that led to dark matter by modifying various forms of gravity. They all work less well than general relativity. So the search continues: keep adjust the models but also don't be afraid that there are new things in the universe that we didn't previously know about. There are several experimental approaches that are searching for ways to directly detect dark matter.

[Maursault]

> People have tried several times to explain the observations that led to dark matter

The observations that led to Dark Matter have been sufficiently explained without the need for Dark Matter.[1] Apparently, subsequent observations that got lumped into Dark Matter have not yet been sufficiently explained, and Science is paradigmatic, so Dark Matter persists.

[1] https://www.youtube.com/watch?v=PL0ewiwqoTw