Does dark matter really exist or is just a convenient invention to explain why some of the equations don't equate? A bit like Einstein's cosmological constant or the luminiferous ether - both eventually discredited when we got smarter.

Much like God; it remains in our discourse because though not conclusively proven, it has yet to be disproven.

At the moment, Ray, it seems like dark matter may be in a different category.

A quote from this link: https://spaceplace.nasa.gov/review/dr-marc-space/dark-matter.html .

..When astronomers look carefully at a galaxy, they can measure how fast the stars within it are moving. The motions of the stars are the result of the gravitational forces from all the other matter in the galaxy. But here is the key problem: When astronomers add up all the matter in all the stars and gas and dust visible with all different kinds of telescopes, it doesn't total nearly enough to explain the motions they observe. The stars are moving around much faster than they should be! In other words, all the matter we can see is not enough to produce the gravity that is pulling things around. This problem shows up over and over again almost wherever we look in the universe. Not only do stars in galaxies move around faster than expected, but galaxies within groups of galaxies do too. In all cases, there must be something else there, something we can't see, something dark.

This mismatch between what we see and what we know must be there may seem very mysterious, but it is not hard to imagine. You know that people can't float in mid-air, so if you saw what looked like a man doing just that, you would know right away that there must be wires holding him up, even if you couldn't see them.

Interacting galaxies.

The name scientists have given to the missing material is "dark matter." We can see the bright matter, like stars, but we know some other matter is there because of how it pulls on the bright matter. The black background of space that we tend to ignore when we enjoy the beautiful sights of the night sky really isn't as empty as you might think. As surprising as it might seem, there is more than 50 times more dark matter than bright matter in the universe.

But what exactly is the dark matter made of? Black holes and objects just a little too small to be stars (so they can't produce light of their own) make up some of it. But all of that adds up to less than one-fifth the total amount of dark matter that must be out there. We believe most of the dark matter is composed of new particles smaller than atoms that are different from anything scientists have ever detected and studied. It seems that the very essence of most of the matter in the universe is different from what you and I and rocks and trees and Earth and the Sun are made of.

I don't know, it's mind-boggling just thinking about it.