Editing Space Interferometry Mission (section)

===Dark matter===
[[File:UniverseComposition.svg|thumb|right|The gray portion of this pie graph shows the estimated distribution of dark matter in the universe.]]
[[Dark matter]] is the matter in the universe that cannot be seen. Because of the gravitational effect it exerts on stars and galaxies, scientists know that approximately 80% of the matter in the universe is dark matter.<ref name=dark1/><ref name=dark2/> The spatial distribution of [[dark matter]] in the universe is largely unknown; SIM Lite would have helped scientists answer to this question.

The strongest evidence for dark matter comes from galactic motion.<ref name=dark1/><ref name=dark2/> Galaxies rotate much faster than the amount of visible [[matter]] suggests they should; the gravity from the ordinary matter is not enough to hold the galaxy together. Scientists theorize that the galaxy is held together by huge quantities of dark matter.<ref name=dark1/><ref name=dark2/> Similarly, clusters of galaxies do not appear to have enough visible matter to gravitationally balance the high speed motions of their component galaxies.

Besides measuring stellar motions within the Milky Way, SIM Lite was to measure the internal and average galactic motion of some of the neighboring galaxies near the Milky Way.<ref name=dark1/><ref name=dark2/> The telescope's measurements were to be used in conjunction with other, currently available, data to provide astronomers with the first total mass measurements of individual galaxies. These numbers would enable scientists to estimate the spatial distribution of dark matter in the local group of galaxies, and by extension, throughout the universe.<ref name=dark1/><ref name=dark2/>