After anextensive search, astronomers say they have definitely found half of theuniverse's missing normal matter in the spaces between galaxies.
Astronomershave long known that the amountof matter we can see doesn't match up with what's actually there. Normalmatter (which includes galaxies, stars and us) makes up only about 4 percent ofthe universe. This type of matter is also called "baryonic" becauseit is made of baryons (protons, neutrons and other subatomic particles).
The missingpart of baryonic matter has largely escaped detection because it is too hot tobe seen in visible light but too cool to be seen in X-rays. Dubbed the"intergalactic medium," or IGM, it extends essentially throughout allof space like a cosmic spider web.
(Thismissing matter is not to be confused with darkmatter, an exotic form of matter that can only be detected by its gravitationalpull.)
A team ofastronomers from the University of Colorado in Boulder used the light fromdistant quasars (the bright cores of galaxies with active black holes) to probethe almost-invisible web-like structure, like shining a flashlight through afog. Their results are detailed in the May 20 issue of the AstrophysicalJournal.
UsingHubble's Space Telescope Imaging Spectrograph (STIS) and NASA's Far UltravioletSpectroscopic Explorer (FUSE), the astronomers found the spectral"fingerprints" of highly ionized hydrogen and oxygen, thought to formthe IGM.
"Wethink we are seeing the strands of a web-like structure that forms the backboneof the universe," said study team member Mike Shull. "What we areconfirming in detail is that intergalactic space, which intuitively might seemto be empty, is in fact the reservoir for most of the normal, baryonic matterin the universe."
Anothergroup of astronomers recentlyfound another filament of the missing baryonic matter connecting twodistant galaxies.
The CosmicOrigins Spectrograph, to be installed on Hubble by astronauts later this year,will help search for weaker signals of this missing matter.
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