Remember, when the facts don't match your theory
Correct. The facts don't match the theory. Galaxies could not hang together the way they do if all they consist of is the things we've already observed in the laboratory unless we change the law of gravity [to something enormously more complex - c.f. epicycles] or postulate the existence of something that interacts gravitationally but doesn't interact with light.
too bad for the facts
I don't get this. Nobody is ignoring the facts. That's why we need to change things. Dark matter is the heliocentric solution. No dark matter is the epicyclic solution.
The [difference between facts and] theory is easily explained
Don't know about easily. There's been a lot of "dark matter" theories that have fallen due to one or more inconsistencies with known physics.
by an abundant (making up 96% of the universe),
Necessary otherwise we have to change the theory of gravity
If by invisible you mean doesn't interact with EM radiation then yes, this is required by the facts.
It's not undetectable. If it were undetectable then we wouldn't need it. It's very detectable - its gravity is what makes galaxies hang together. Its gravity is what allows gravitational lensing to happen where there isn't any (visible) matter to make it happen.
Definitely not magic. It has to agree with all the laws of physics. Conservation of momemtum, conservation of energy, speed of light etc.
it's called dark matter because it doesn't interact with the EM spectrum. It neither emits EM radiation nor absorbs it.
that is everywhere and affects everything.
Actually, I think this is one of the great unknowns. Whether it's large numbers of light particles or smaller numbers of massive particles. Its primary interaction with the known universe is through gravity which yes, does affect everything, everywhere, at the speed of light.