Of course, it could be that dark matter does not exist. In a very real way, nothing in physics “exists” because like all natural sciences physics does not make statements of objective truth, it makes testable predictions.

Dark matter, string theory, aether, etc., those are models that we, at some point in time, think may help us get better predictions and design further experiments. All models turn out to be wrong in the end, but they can be helpful until we come up with better ones.

If you drop the dark matter model, then you would want to have some other model as for why we observe what we observe. Some people find that other available models are even worse than the dark matter one, but if you don’t think so you can take your pick.