Not true at all, there are 4 main observational pieces f evidence to suggets dark matter exists. Is it too hard to have a look at wikipedia before making a straw man argument up?
Dark matter - Wikipedia, the free encyclopedia
Furthermore, sceintist have cosnidered re wrtining one of their most cherished equations:
f=ma
to try and account for these observations with envokig the existence of dark matter, These attempts are known as MOND:
Modified Newtonian dynamics - Wikipedia, the free encyclopedia
However no one come up with a MOND that can be shown to work.
Can you imagine religious people throwing out key doctrines if they dont match observations?