What can the answer be? Is it possible that most galaxies are surrounded by some 'dark' form of matter that cannot be observed by radio, infrared, optical, ultraviolet, X-ray, or gamma-ray telescopes? Could Einstein's theory of gravity, which has proved to be correct in all cases so far, be somehow wrong?
X-ray telescopes have discovered vast clouds of multimillion degree gas in clusters of galaxies. These hot gas clouds increase the mass of the cluster, but not enough to solve the mystery. In fact they provide an independent measurement of dark matter. The measurement shows that there must be at least four times as much dark matter as all the stars and gas we observe, or the hot gas would escape the cluster.
'We are all in the gutter, but some of us are looking at the stars.'