You should look into MOND.
"Dark matter" as an effect is very well established. It is a sign of a failure in our models of physics. That failure could be in the microphysics (thus, various particle models, such as WIMPs), or in the macrophysics (i.e., in general relativity, the model for gravity, which is modified by theories such as MOND).
Now, as it happens, these sorts of galaxy cluster collision observations are probably the strongest test of MOND type theories - it is hard to see how a failure of gravity would get separated from the matter causing the gravity. MOND is not yet firmly ruled out, but it does look a lot less plausible.