Because every other observation for hundreds of years is explained perfectly by "The equations", so you need to "Fix" them in a way that doesn't change any of the existing answers.

And yes, as Cozzyd points out, matter is actually part of these equations. The simplest changes involve adding an extra term to the equation with such-and-such properties that disappear in the normal cases - that is dark matter.

You don't comprehend the magnitude of "just fixing the equations".

As other commenters said, physics has a history of "adding unobservable objects", and then finding a way to observe them.

We already observe it via its gravitational effects on galaxies. Additionally, good luck getting "fixed equations" to agree with the reams of data.

Isn't that what MOND is doing, to some degree?

From what I have looked into, it breaks down with observed things on the fringes, aka black holes and supercluster type stuff.

This is the "silver bullet" if you will for a lot of these theories: https://en.wikipedia.org/wiki/Bullet_Cluster

It would be great if "just" modifying the equations works. But it has an uphill climb against observed data.

Adding a particle is "just" fixing another equation (the standard model Lagrangian).