After an extensive search, astronomers say they have definitely found half of the universe's missing normal matter in the spaces between galaxies.

Astronomers have long known that the amount of matter we can see doesn't match up with what's actually there. Normal matter (which includes galaxies, stars and us) makes up only about 4 percent of the universe. This type of matter is also called "baryonic" because it is made of baryons (protons, neutrons and other subatomic particles).

The missing part of baryonic matter has largely escaped detection because it is too hot to be seen in visible light but too cool to be seen in X-rays. Dubbed the "intergalactic medium," or IGM, it extends essentially throughout all of space like a cosmic spider web.

(This missing matter is not to be confused with dark matter, an exotic form of matter that can only be detected by its gravitational pull.)

A team of astronomers from the University of Colorado in Boulder used the light from distant quasars (the bright cores of galaxies with active black holes) to probe the almost-invisible web-like structure, like shining a flashlight through a fog. Their results are detailed in the May 20 issue of the Astrophysical Journal.

Using Hubble's Space Telescope Imaging Spectrograph (STIS) and NASA's Far Ultraviolet Spectroscopic Explorer (FUSE), the astronomers found the spectral "fingerprints" of highly ionized hydrogen and oxygen, thought to form the IGM.

"We think we are seeing the strands of a web-like structure that forms the backbone of the universe," said study team member Mike Shull. "What we are confirming in detail is that intergalactic space, which intuitively might seem to be empty, is in fact the reservoir for most of the normal, baryonic matter in the universe."

Another group of astronomers recently found another filament of the missing baryonic matter connecting two distant galaxies.

The Cosmic Origins Spectrograph, to be installed on Hubble by astronauts later this year, will help search for weaker signals of this missing matter.