[ben_w]

It has been observed, by its effect on other things, and in particular because this effect isn’t always the same. Most galaxies’ stars orbit at the wrong speed, but some behave as expected. Most galaxies gravitationally lens the background more than expected, but in some cases (colliding groups) something that doesn’t shine or reflect any EM radiation is causing lensing next to the galaxy while the galaxy itself is closer to what we expect without dark matter.

[Flankk]

That's right, observations were made that do not fit with our current understanding of physics (equations). That means our understanding is wrong. So instead of assuming the equations are wrong physicists invented an invisible type of matter. That is not science. Dark matter is nothing more than modern day aether.

[regularfry]

If you wanted to get particularly philosophical, everything is observed by its effect on other things.

[ben_w]

Sure, but my point was that dark matter is not just an artefact of equations.