[sidek]

But the motivation behind dark matter is deeper than its gravitational observations. For instance, almost all particles physicists believe the standard model of particles is incomplete. No one is really sure exactly what we need to add to the standard model, but it turns out that if you add things to it in any sensible way, you almost always get things that look like they could explain dark matter.

So, it's more: particle physicists all believe the real theory has this aspect; when we look at gravity it looks like it had this aspect; therefore we should probably find a way to modify our theory so it has this aspect.

The major reason we haven't been able to go further than this is because we haven't been able to pin down exactly what these new particles are. Particle experiments are /hard/ (see:LHC), and it might take a long time.