Benjamin Floyd

At the center of a vast city of galaxies hangs a thick cloud of gas. It has cooled to the point where, by all the rules, it should be collapsing into new stars, like steam turning to droplets on a cold window. But the window stays dry. Astronomers aimed the Chandra X-ray telescope at this cloud and watched it for nearly three straight days.

Astronomers used the infrared telescope and the old Hubble to discover a gaping void in the center of a galaxy in the Abell 402 cluster, as if an invisible tugboat had pulled billions of stars out of there. The missing mass equals two billion suns: it's not a cloud but a real lack of stars.

Two astronomers look at the same universe but see it differently. One measures the expansion rate using ancient light that has been traveling toward us for nearly 14 billion years. The other uses recent stellar explosions in nearby galaxies. The first gets 67 kilometers per second per megaparsec, the second gets 73.