There are dark lines in space, and they hide in the light. We've known that they're there for nearly a hundred years, but we still don't know what they actually are. All we know is what we call them - diffuse interstellar bands - and that they mean that something is out there in space that we haven't identified.
At the start of the roaring twenties, Mary Lea Heger spotted the first example of what has become an enduring mystery. She noticed that a binary star system's spectrum of light — the rainbow-like range of wavelengths of light emitted by bright objects — had gone dark in a couple of places. This wasn't unusual. These were absorption lines. Certain atoms or molecules absorb certain wavelengths of light while letting other wavelengths through. These absorbed wavelengths show up as dark lines on the continuum of color in a spectrum.
What was unusual was these lines weren't shifting. Generally, in a binary system, the stars are moving toward and away from the viewer on Earth. As they move, their spectrum lines shift, much the way the sound of a car horn shifts if it's moving towards or away from you. These lines weren't shifting.
After some analysis, it was discovered that the lines weren't shifting because they came from material that was between the stars, not part of them, but to this day, no one has been able to lock down what exactly these lines are. The absorption lines, called diffuse interstellar bands, or DIBs, pop out all over the electromagnetic spectrum. There are hundreds of them in space. Generally, this would be helpful information. Scientists use absorption bands just like these to determine what atoms are present in space. Different atoms emit or absorb different wavelengths of light. By looking at what emission or absorption lines show up on the spectrum of light given off by celestial objects, scientists know that an exoplanet's atmosphere contains, for example, oxygen, or nitrogen.
These lines are different. Scientists think these absorption lines are the sign of complex molecules. Atoms give off light when their electrons shift from one orbit to another and then fall back down. It's a simple and defined process. Molecules are made up of many different atoms, and those atoms crowd close to each other in ways that make one atom interfere with the normal spectrum of another. What's more, both the atoms in molecules and the molecules themselves, wiggle around. Their absorption and emission lines depend on their inner and outer position. Nobody has found any that can exactly correspond to the DIBs.
So what could the DIBs be? They could be ordinary substances whose spectrum lines are changed by the unique conditions of deep space. They could be tiny diamonds. They could be anything from carbon nanotubes to alien bacteria. We don't know. All we know is they're definitely up there. And they're dark.
Triangulum Galaxy Image: NASA/Swift Science Team/Stefan Immler