disk of hydrogen atoms approximately 100 million times the mass of the Sun. When they gauged the rotational speed of the pancake-shaped entity, it presented itself as if it were actually 1,000 times heftier—that is, 100 billion times the Sun’s mass. Consequently, the astronomers concluded that more than 99.9 percent of the body is composed of unseen material. Observations shortly thereafter at the Arecibo radio observatory in Puerto Rico confirmed the Jodrell Bank picture. A close examination of the dark colossus found absolutely no trace of stellar objects. Hence, VIRGOHI21 is the first-known completely starless galaxy.

The Cardiff team has speculated about the invisible galaxy’s origins. One possibility the team investigated was that it consists of material wrested from other galaxies in a cosmic tug-of-war. However, no nearby galaxies stood in the proper positions for exerting such tidal forces. “If it is tidal debris,” Minchin and his colleagues concluded, “then the putative parents have vanished.”

Another possibility the team investigated is that VIRGOHI21 is a gravitationally bound system whose hydrogen is too dispersed to clump into stars. Its scattered pockets of hydrogen may lack the critical density to fuse together and burn. Given the data the researchers found, this seemed to them the most likely option. Using their discovery as a model, they launched a concerted effort to find other invisible galaxies in space, for although VIRGOHI21 is the first-known completely dark galaxy, other galaxies brim with unseen material. This matter forms dark halos around the shining stars, protruding far beyond the visible bounds of these objects. The nature of this dark substance is largely unknown—a long-standing astronomical mystery.

Astronomers have suspected for many decades that the visible content of the universe falls far short of the amount that is apparently exerting gravitational force. Hints of this missing mass conundrum date as far back as the early 1930s, when (as we discussed) astronomers Jan Oort and Fritz Zwicky found unexplained behavior



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