Up to 90% of the matter in galaxies cannot be seen by optical telescopes.
This "invisible" material is usually either too cold to radiate enough visible
light to be detected, or is hidden behind thick interstellar dust.
Infrared light, however, can detect cool objects in space and can
penetrate regions of thick dust.
Infrared studies have shown that a possible candidate for this
"missing mass", which cannot be seen in visible light, is the cold, invisible
matter which makes up massive molecular
clouds in the disks of galaxies.
The image to the right shows the infrared signatures of molecular hydrogen
gas (H2) in the central dust lane of the edge-on spiral galaxy NGC 891.
The gas temperature determined from this spectra is about 80 degrees Kelvin.
In addition, the molecular hydrogen is the amount needed to account
for this galaxy's missing mass.
ISO-SWS data from Valentijn and van der Werf / SRON.
Optical photograph of NGC 891: Blair Savage, Chris Howk (U. Wisconsin)/N.A.Sharp (NOAO)/AURA/NSF