Astronomers have known for a long time that the universe is expanding from the initial Big Bang. They had 3 models for expansion: the universe would continue to expand forever, the universe would expand and then slow to a stop, the universe would slow, stop, and then retract, into a huge collapse initiating another Big Bang.
Along comes the Hubble Space Telescope, to change all that. From Hubble, astronomers viewed things in the universe that completely surprised them. They witnessed galaxies that are moving away from one another, in other words, the universe is expanding, but they determined that the universe expansion is speeding up, not slowing down.
This was completely unexpected, because our knowledge of gravity suggests that large objects in the universe should "attract" one another. The observations of galaxies expanding at a greater rate now as compared to billions of years ago, suggests there is something else out there that is causing this increased rate of expansion.
That "something" is dark matter and dark energy. The term dark is used because we cannot see it using ANY light. It does not reflect, does not refract, does not absorb light of any wavelength (visible light, infra-red, X-rays, gamma rays, ultraviolet light and radio waves).
These lights are the only tools of the astronomer, so if something does not interact with any of these types of light, it cannot be seen! However, the Hubble Space telescope provided astronomers with something never seen before, but was actually predicted by Einstein.
You see, Einstein predicted that the immense gravity of very large objects in space actually warp space and time, but they also bend light! This is shown by images produced by Hubble (below) compliments of NASA.
Light from distant galaxies is "reflected" by other immense objects that are in the "direct line of sight" producing additional images of these distant galaxies. These images are the result of the bending of the distant light and this bending is called gravitational lensing. Something very massive is bending light from entire galaxies and that something is theorized to be dark matter.
Throughgravitational lensing Hubble sees 2 images for the distant galaxy. The immense red "cloud" surrounding the galaxy cluster is thought to be dark matter which bends the distant light. The bending of light into these additional images is called gravitational lensing, and that is how astronomers know that dark matter exists.
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