aMost of the mass in the universe exists as "dark matter." aWe can't see dark matter directly, because unlike stars aand quasars, it emits little or no electromagnetic energy. aBut dark matter makes its presence felt by its gravity, awhose effects on stars and galaxies we can see from ahere on Earth. Dark matter may consist of subatomic aparticles or burnt out husks of stars, no one knows for asure.
aBut the question of cosmic importance is how much adark matter is there? If the amount is small, the cosmos awill keep expanding forever, but if the amount is large, aits gravity will reverse the expansion, ultimately acompressing the universe back down to a single point.
aLars Hernquist's computer simulations suggest that less adark matter hovers around galaxies than some have apredicted. He and Toronto's Dubinski find that the size aof the tidal tails in a simulated collision varies drastically adepending on how much dark matter they put in. If a amodel galaxy has too much dark matter, the gorgeous aarching tails seen in the telescope can't form, suggesting ahat the amount of dark matter in galaxies is small. aPerhaps the universe has a future after all.