There’s A Debate Raging Over Whether Dark Matter Is Real, But One Side Is Cheating
“On the scales of groups of galaxies, individual galaxy clusters, colliding galaxy clusters, the cosmic web, and the leftover radiation from the Big Bang, MOND’s predictions fail to match reality, whereas dark matter succeeds spectacularly. It’s possible, and perhaps even likely, that someday we will understand enough about dark matter to understand why and how the MOND phenomenon on the scales of individual galaxies arises. But when you look at the full suite of evidence, dark matter is practically a scientific certainty. It’s only if you ignore all of modern cosmology that the modified gravity alternative looks viable. Selectively ignoring the robust evidence that contradicts you may win you a debate in the eyes of the general public. But in the scientific realm, the evidence has already decided the matter, and 5/6ths of it is dark.”
Look at a galaxy, watch it rotate, and be extremely careful. Look at how the rotation speeds vary as you move farther and farther away from the center. And then look at all the matter you can see: gas, dust, stars, black holes, neutron stars, plasma, etc. Does the matter describe the galactic motions you see? No, no it doesn’t. Immediately, there are probably two potential solutions that pop into your head:
1.) Either there’s some additional, invisible, unseen mass, like dark matter, or,
2.) The law of gravity that you’re using is wrong.
Both of these seem like reasonable options, and if all you had was individual galaxies, they both would be.