That’s exactly what Dark Matter is. Scientists saw that galaxies were spinning faster than expected, did some math, and figured out that based on current, known physics, they wouldn’t be able to stay together.
So they said “huh, must be additional matter that we can’t account for, let’s call it Dark Matter for the time being, cause we can’t see it.” It might be one big type of thing, it might be a thousand smaller types of things that all add up to this collective Dark Matter, but whatever it is, it doesn’t behave the same way we expect normal, everyday matter behaves.
Other scientists said that we must not understand something about physics and gravity at larger scales.
Other scientists said that light must not act the way we expect, and it’s throwing off our measurements.
Based on follow up research, there is more evidence for unaccounted for matter, than the other options.
It’s entirely possible that none of those options are correct, but most of the data we have right now points to Dark Matter is the best fit for the evidence we have.