Here goes… Strap in!
Dark Matter. It’s not “dark” for a start. It appears (maybe) to be simply something which either doesn’t interact with the rest of the Universe except by gravity, or only “weakly” interacts with the rest of the Universe.
A little explanation… Since the early 1930’s Astronomers have found (and continue to find) discrepancies in the way distant Galaxies behave.
Dark matter was first proposed in 1933 by Swiss-American astronomer Fritz Zwicky, who observed that galaxies in the Coma cluster were moving too quickly to be held together by the visible matter alone. His findings suggested the existence of unseen mass, which he referred to as “dark matter.”
Fast-forward to the 1970’s and Vera C. Rubin and colleagues were the first to discover that something weird was going on with the way Galaxies rotate. They started with the Andromeda Galaxy and followed up from there over the years. From her page at UCLA I’ve linked above: “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions,” Astrophysical Journal 159: 379 (1970), with W. K. Ford, Jr..
Sidenote: Vera Rubin was the FIRST lady EVER allowed to observe in the Palomar Observatory in 1965.
Over the years, the problem became more and more overwhelming and annoying. The sums didn’t add up. The more Astronomers looked, the more the Universe didn’t behave the way we think Gravity says it should.
The sentence: “Measured orbital velocities for hydrogen clouds in over 60 galaxies and found the velocity does not decrease at large distances from the center as would be expected if the centripetal acceleration were due to the gravitational pull of luminous matter in the galaxy.” describes it.
Eventually, Astronomers grudgingly (and I mean VERY grudgingly) mostly agreed upon, ish was that there must be some gravity missing. It has so far taken 92 years since Zwicky initially proposed the possible existence of Dark Matter to the majority acceptance that it is the only possible explanation. For the last 30-40 years (ish) the race has been on to figure out exactly what this stuff is. Is it just gravity? Is it a particle? Is it “normal” baryonic matter? What is it?
In Summary: We measured stuff, the sums didn’t add up, we came up with a weird name to call the stuff we assume is there because we can see it’s gravitational influence in the Universe everywhere. 26% of the total existing matter in the Universe is thought to be Dark Matter. The most distressing part about this story is that no matter (pun intended) what we do, so far NO ONE has actually managed to find any trace of any particle of whatever Dark Matter is made of. There are hugely sensitive (expensive) detectors & teams of scientists attempting to find something which might be Dark Matter. We think it’s everywhere… We might be swimming in Dark matter right now! All that money, time, effort, people, and so far, nothing. Imagine devoting your entire life to something that no-one has ever actually seen? I wouldn’t have the patience.
Even shorter summary: Most of our Universe is missing and we don’t know if it’s name is “Frankie” or not, let alone what colour their hair is!
A great in-depth zip through the subject here: https://astronomyexplained.com/history-of-dark-matter-a-journey-through-time/