Astronomers may have underestimated the tally of galaxies in some parts of the Universe by as much as 90 percent, according to a study reported on Wednesday in Nature, the weekly British science journal.

Surveys of the cosmos are based on a signature of ultraviolet light that turns out to be a poor indicator of what's out there, its authors say.

In the case of very distant, old galaxies, the telltale light may not reach Earth as it is blocked by interstellar clouds of dust and gas -- and, as a result, these galaxies are missed by the map-makers.

"Astronomers always knew they were missing some fraction of the galaxies... but for the first time we now have a measurement. The number of missed galaxies is substantial," said Matthew Hayes of the University of Geneva's observatory, who led the investigation.

Hayes' team used the world's most advanced optical instrument -- Europe's Very Large Telescope (VLT) in Chile, which has four 8.2-metre (26.65-feet) behemoths -- to carry out the experiment.

They turned two of the giants towards a well-studied area of deep space called the GOODS-South field.

The astronomers carried out two sets of observations in the same region, hunting for light emitted by galaxies born 10 billion years ago.

The first looked for so-called Lyman-alpha light, the classic telltale used to compile cosmic maps, named after its US discoverer, Theodore Lyman. Lyman-alpha is energy released by excited hydrogen atoms.

The second observation used a special camera called HAWK-1 to look for a signature emitted at a different wavelength, also by glowing hydrogen, which is known as the hydrogen-alpha (or H-alpha) line.

The second sweep yielded a whole bagful of light sources that had not been spotted using the Lyman-alpha technique.

They include some of the faintest galaxies ever found, forged at a time when the Universe was just a child.

The astronomers conclude that Lyman-alpha surveys may only spot just a tiny number of the total light emitted from far galaxies. Astonishingly, as many as 90 percent of such distant galaxies may go unseen in these exercises.

"If there are 10 galaxies seen, there could be a hundred there," said Hayes.

The discovery could add powerfully to knowledge about the timeline by which stars and then galaxies formed.

"Now that we know how much light we've been missing, we can start to create far more accurate representations of the cosmos, understanding better how quickly stars have formed at different times in the life of the Universe," co-author Miguel Mas-Hesse said in a press release issued by the European Southern Observatory (ESO).

Only a small part of the light spectrum is visible to the human eye, which is why astronomers use ultraviolet, gamma and other radiation sources as additional sources for observation.