In 1961, the Swedish astronomer Bengt Westerlund was taking photographic plates of the sky near or on the plane of the Milky Way — that is, looking deep into the murk of our flattened disk of a galaxy. He was observing at Mt. Stromlo in Australia, where the Milky Way rises high above the horizon, giving fantastic views of the galaxy.
In the images he found something very interesting: A dense cluster of stars, heavily obscured by clouds of dust between us and it. Cold interstellar dust (located far away from and between stars in the galaxy) is made of tiny grains of rocky or carbonaceous material, and is excellent at absorbing light. It not only makes objects behind it look fainter, but it also reddens the light, in the same way the Sun looks redder when it’s near the horizon (astronomers call this extinction).