Image: Nature

There is a lot of matter out there in the universe, but most of it is invisible. An international team of researchers, however, has further investigated just how much exactly can or can't be seen, and they report their findings in this week's Nature.

Using a telescope near Coonabarabran in eastern Australia, the scientists have been creating a vast map of the universecharting over 150,000 galaxies so far (see image). By the end of the year, they expect to have mapped 250,000 galaxies, making this survey 10 times larger than the largest of its kind. "We use the galaxies as a tracer of mass in the universe," explained Richard Ellis of the California Institute of Technology. The team found that 90 percent of the galaxies they observed were distributed on the surfaces of big bubbles of seemingly empty space. The theory is that denser regions of the expanding universe are slowed down by gravity more than their surroundings, which creates areas with increasingly higher or lower density.

The researchers used their map of galaxy distribution to measure the total mass density of the universein other words, how much of the universe is mass. They concluded that about 35 percent of the universe's energy exists in the form of matter, and about 65 percent in the form of "vacuum energy," or dark energy. This dark matter is invisible to us and thus far can't be detected but is believed to be pushing the universe apart.