Last week, the Arecibo radio telescope in Puerto Rico began a comprehensive survey (dubbed ALFALFA, for Arecibo Legacy Fast ALFA Survey) of all galaxies in a large area of the sky to a distance of 800 million light years. This search might very well discover some of the so-called dark galaxies, which have never been seen. Such galaxies (which exist only in theory) would compose of huge amounts of hydrogen and helium, but have never ignited to form stars (or the stars perhaps are hidden behind a dense fog of dark matter). Such cold matter would only be visible at radio-frequencies, whereas optical (such as Hubble) and x-ray (Chandra) telescopes will not be able to see them.

Arecibo dish(Courtesy: National Astronomy and Ionosphere Center)The telescope is fitted with a new compound eye. Last year, its sensitivity was further boosted by the Arecibo L-Band Feed Array (ALFA): essentially a seven-pixel camera that will allow astronomers to collect data about seven times faster than before.
Such dark galaxies might be responsible for a large portion of the missing matter in the universe. Accounting for this matter will help us better understand the large scale structure of the universe, and also confirm/negate theoretical models/predictions regarding the creation and fate of our universe.