They did try. Those alternative theories fall under an umbrella called MOND- MOdified Newtonian Dynamics. They were actually some of the first real proposals to

They did try. Those alternative theories fall under an umbrella called MOND- MOdified Newtonian Dynamics. They were actually some of the first real proposals to solve the dark matter problem. Unfortunately, none of these theories panned out. The easiest to understand evidence against MOND is shown in the bullet cluster. The bullet cluster is a pair of galaxies in the process of colliding. They also happen to be gravitationally lensing galaxies behind them. While the xray and visible light observations show that gas clouds that make up the bulk of the normal matter in the galaxies stopped when they hit each other, (like two cars in a head on collision) the gravitational lensing shows that the overwhelming majority of the mass in the galaxies simply passed straight through, the way dark matter as particles would. So in order to accommodate the bullet cluster, MOND will need to show that empty space has some sort of momentum and memory, which is going to be much less elegant than a second hierarchy of particles which do not interact via the electromagnetic or strong nuclear forces. There are a very large number of other observations which also disprove MOND or variations on them, the bullet cluster is just the easiest to describe in a paragraph. It's actually very, very common for laypeople to come up with MOND as a solution for dark matter. I'm pretty sure I did when I first heard about dark matter. I don't think I've ever seen a dark matter post on HN or reddit that didn't include at least one post positing MOND, even if (especially if) they don't that the theory they came up with has a name, an acronym, and had already been thoroughly disproven by the scientific community. https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster