... The Coma Cluster of Galaxies . ... Deep inside Centaurus A, the closest active galaxy to Earth, lies ... another galaxy! ... At least that was the conventional wisdom when it was first realized that the space between the stars is filled with hydrogen, a strong absorber of extreme ultraviolet light. ... Is the Moon larger when near the horizon? ... The result, shown above, is the most complete sky map of the most common visible light emitted from the most abundant element in our Galaxy: hydrogen. ...