A sentence or two? Jeez, whole books have been written about it...

For instance, there's
In Search of Dark Matter by Ken Freeman and Geoff McNamara, published last year, which I'm currently about halfway through reading. That one says that about 96% of the material in the universe is dark matter and energy (I've seen other--and older--estimates in the 70-80% range), detectable largely by its gravitational influences on other galaxies. Nobody knows quite what it is, whether it's hot or cold, or even exactly where it is, though the evidence suggests it's in halos surrounding galaxies, not within the interstellar spaces in the galaxies themselves. The observational evidence is subject to large uncertainties, as it's based largely on observed rotation rates of stars around galaxies and the behavior of galaxies in clusters. That's accelerated motion, according to both Newton and Einstein, and the observed accelerations cannot be accounted for by the amount of visible matter: galaxies ought to be flying apart. The accelerations, however, are *very* small, and another possibility is that there's no dark matter or energy at all, it's just that Newton's and Einstein's equations break down at very small accelerations, in much the same way Einstein showed Newton's do at very large ones. That's the argument of last resort though; physicists are extremely reluctant to give up theories that have worked so well for so long. At some point though, they'll have to, because relativity and quantum theory are fundamentally inconsistent; they can't both be right, in their present form. String theory at the moment offers the best hope of unifying them.
Mysticism, just to tie up any possible loose ends, offers nothing.