科学美国人60秒:Dark Matter Dominates Just-Discovered Galaxies

Astronomers have discovered a trove of galaxies that are virtually invisible—because they’re made almost entirely of dark matter. The Subaru telescope in Hawaii spotted 854 of these oddballs, which are referred to as “ultradiffuse galaxies,” by detecting what little light they do produce. They were all found in what’s called the Coma Cluster of galaxies. The report is in The Astrophysical Journal Letters. [Jin Koda et al, Approximately a Thousand Ultradiffuse Galaxies in the Coma Cluster]

Of course, scientists still do not know just what dark matter is. But they can detect its presence through its gravitational effects on the normal matter that we can see. That’s how we know that dark matter seems to be ubiquitous in the universe—especially in these newly found, barely visible galaxies.

Many of these galaxies are about the size of our Milky Way, but contain just a thousandth as many stars. Researchers estimate that dark matter accounts for 99 percent of these galaxies’ mass.

How such objects could form is a mystery. They probably started out with a healthy complement of star-forming gas, just like normal galaxies, but somehow lost it. The gas might have blown away as the galaxies moved through the Coma Cluster, or maybe other galaxies’ gravity dragged it off. Further study of these ultradiffuse galaxies should clarify the situation, and may even shine some light on the fundamental nature of dark matter.

—Clara Moskowitz