The Expanding Universe
One hundred years ago, back in the 1910s scientists, came to the crazy realization that the universe is expanding. In 1912, Vesto Slipher discovered that the light from certain groups of stars (they later found those stars were galaxies) was redshifting away from their telescopes. Red Shifting means the wavelength of the light - stretched, so the light is seen as 'shifted.
The redshifting indicated the galaxies must be moving away from the Earth. Vesto Slipher finding resulted in astronomers beginning to measure the redshifts and distances to galaxies. Astronomers concluded the superior the redshift, the faster the galaxies were moving away from Earth.
Some people think that everything that is redshifting is moving away from us or everything else. However, everything isn't moving away from us, rather-space is stretching it's self out. This phenomenon is occurring because of dark energy. Dark Energy is a hypothetical form of energy that exerts a negative, repulsive pressure, behaving like the opposite of gravity.