[Avshalom]

So basically there are four forces: weak and strong nuclear forces; gravity and EM.

EM is mediated by photons and visible matter is constantly absorbing and emitting photons.

Some particles don't though. Neutrinos for instance. They're dark, they don't interact with light.

So when observation showed that there needs to be more matter laying around than we could see some enterprising young turks said "hey, maybe there's a shit ton of dark matter" and that makes the math come out right.

the obsession is mostly because people who aren't astronomers seem to think "dark" is a metaphor and go "aha now that no one cares about the oxford comma this will be my new opinion"

Up for debate is what kind of particles make up the dark matter and how much mass is dark vs just poor measurements and/or some tweaks to gravity, but unless some one manages to disprove neutrinos: dark matter exists.