Dark matter

What is dark matter? Why do we believe galaxy clusters are rich in dark matter? Some matters do not emit and reflect lights. Thus they cannot be seen or detected. Existence of this invisible matter is concluded from rotation curves and from other techniques. The existence of this dark matter is not conformed by the observation of any electromagnetic wavelength. Dark matter is believed to constitute about 90% of the matter in the universe.  The high velocity within the clusters of the galaxies and the gravitational lensing indicates the presence of dark matter in the galaxy clusters.