Dark matter is really not such an exotic idea.  In fact, theories that try to
explain the gravitational and energetic evidence for dark matter with only
"ordinary" matter are far more convoluted and outlandish than the existence
of dark matter itself.  Nevertheless, there is something unsettling about
the realization that the properties of the vast majority of matter in the universe
are completely unknown.  By studying how much dark matter exists, and how it's
all distributed in the universe, scientists hope to make some progress toward
understanding this vast frontier of astrophysics.