The Expansion of the Universe
A favorite occupation of observational astronomy is to determine the "red shift" of galaxies, a concept introduced by E. P. Hubble. Redshift is defined as the change in frequency of light to lower energy, or longer wavelength, and the name comes from the fact that this phenomenon tends to shift light’s frequency toward the red end of the visible light spectrum. The amount of redshift of the light coming from a luminous object depends on the speed with which it recedes from the observer (that is, us). More distant objects show the larger red shift, from a phenomenon known as the Doppler Effect (more on observational astronomy in the sections below). The conclusion is that the universe is expanding and that, on the whole, galaxies and clusters of galaxies are receding from each other.
Now with the "great eye in the sky", the Hubble space telescope, millions of galaxies can be readily seen. At the same time, the Hubble telescope looks back in time billions of years, when the universe was still younger.
Perhaps the most awesome result of all these studies is the unimaginable vastness of space, its emptiness. Truly, the galaxies are "island universes" as surmised by Immanuel Kant. The "normal" view in space, at an arbitrary position someplace within a hundred million light years, would be a very dark sky (no stars) populated with faint nebulae. The distances are so great that communication between intelligent beings across the abyss of time and space from one galaxy to another, such as are written about in science fiction, seem utterly impossible.