Mysterious Dark Galaxy Puzzles Astronomers

A mysterious dark galaxy continues to puzzle astronomers and the latest pictures taken by Hubble failed to shed some light on the mystery. This galaxy raised many questions about how it formed and its evolutionary path over the history of the universe.

About 300,000 years after the Big Bang, atoms of hydrogen and helium began to form and after some time, larger structures began to appear in this primordial matter. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures would eventually become the galaxies we see today.

The problem is, the mysterious, galaxy-sized cloud of hydrogen known as VIRGOHI21 apparently has almost no stars, making it the only known example of a 'dark galaxy' that never kick-started star birth.

It seems that the galaxy formation process sometimes goes wrong, suggesting that for some unknown reasons, some galaxies are stillborn and simply fail to form stars. The discovery of VIRGOHI21 in 2005 seemed to provide the first evidence that dark galaxies existed.

Hubble looked a patch of space 50,000 by 50,000 light years across, centered on the hydrogen cloud's position and found only 119 red giants, which is approximately the number of stars typically found in a region of the same size, between galaxies.

There was another theory that claimed to have the answer to the mystery. Some scientists suggested that VIRGOHI21 was pulled out of the nearby galaxy NGC 4254 when another galaxy called NGC 4262 shot past it at 900 kilometers per second.

The only problem is that the 119 red stars would be three times fewer than expected if the cloud were a large piece of celestial wreckage. "If the hydrogen in VIRGOHI21 had been pulled out of a nearby galaxy, the same interaction should have pulled out stars as well," says Minchin.

So, for the moment, the mystery continues and as Michael Merrifield of the University of Nottingham in the UK, said, "Even if this is a dark galaxy, it is not what you expect to find".