The kind of light we see is called optical light. It's actually rather low energy, emitted by hot things like the Sun, gas clouds, and so on. But what if we could see light that had energies millions of times higher? Billions?
Then the sky would look like this: a map from NASA's Fermi telescope, which sees in gamma rays. Sources of gamma rays are among the most violent in the Universe: exploding stars, fiercely magnetic neutron stars, black holes gobbling down matter. Fermi just completed its third year in space, surveying the entire sky and building up a large and sensitive database of this highest-energy form of light. While many of the individual sources are identified, as many as one third of all the objects in this map cannot be determined.
And that line across the middle? That's our own galaxy, the Milky Way. It's a flat disk, and we're inside it, so we see it as a broad line across the sky. It takes a dark night to see the faint milky band of the galaxy to the naked eye, giving no real hint of the vast and terrible forces at play there. Only by examining the sky in other energies do we start to unveil the true nature of the Universe.
Credit: NASA/DOE/Fermi LAT Collaboration