One of the most important constraints placed on any scientific theory is it has to be consistent with all of the available evidence and results. Dark matter (DM) was initially inferred from measuring the rotation curves of galaxies, which indicated galaxies had roughly 10x the mass compared to the atoms and ions observed in galaxies. While scientists do not yet know what DM is and alternative explanations have been explored, there are several cosmological pieces of evidence supporting the fact that DM accounts for 90% of the total mass in the universe. Alternative explanations (e.g. modified-Newtonian dynamics or MOND) have not been able to become full-fledged integrated theory. DM remains the only valid theory, at least until all candidate subatomic particles can be ruled out. Only then, will the search for new physics to explain the observations be necessary.
Cosmological arguments begin by noting how extraordinarily smooth the early universe was only a few years after the Big Bang. If you compare it to a sphere, the surface is perfectly smooth to better than 1/100,000. (For reference, the best polished optical surfaces a thousand times less smooth.) In contrast, the current universe is observed to be highly clumpy on all scales, except the very largest scale. All of the highly non-uniform structure seen today had to have grown gravitationally from the miniscule differences in density in the early universe. There isn’t sufficient time for structures to grow into galaxies, clusters of galaxies, and superclusters of galaxies without DM. Moreover, DM is necessary to create the ubiquitous suppermassive black holes (SMBH) in the galaxy cores. SMBH are millions and even billions of times more massive than our Sun. DM is necessary to create a deep enough gravitational well to trigger SMBH formation.
Gravitational lensing studies (both strong and weak) support the evidence for DM. DM in the intergalaxy regions inside clusters of galaxies is necessary to produce multiple images and the associated separations of very distant QSOs and active galaxies. The presence of this extra (missing) mass is inferred from X-ray observations of shocked ionized gas.
Only DM and normal gravity are necessary to explain this vast body of evidence. Put that in your pipe along with some really good weed and smoke it. I’m sorry for the length and technicality of this post, but various posts in the past have poo-pooed DM since we don’t know all there is to know about it.