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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What is Dark Matter and does it produce gravitational lensing?

By measuring the speeds of stars within galaxies, and the speeds of galaxies within distant clusters of galaxies, astronomers have become convinced over the decades that there is a sub-luminous or even non-luminous 'something' which contributes to the gravitational fields of these systems, but which you cannot count-up optically so that the speeds and the masses 'balance' each other. This is 'dark matter' or 'missing mass', and some physicists have proposed it may not even be matter. It could be in the form of massive neutrinos or other exotic particles which are not matter ( neutrons, protons, electrons).

Because dark matter produces gravity, it can produce or enhance the lensing abilities of galaxies or clusters of galaxies in which it resides. In fact, astronomers have for 10 years been studying the lensed images of galaxies seen through distant clusters, and have used the lensing geometry to determine how much mass the intervening cluster must have. The answers always seem to indicate that some additional dark matter is needed; perhaps 2 - 3 times the mass of the visible galaxies in the cluster itself.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.