Contrary to how it looks out in the wilderness, the night sky is not dark. It glows very faintly. Some of this light is sunlight reflected off of the moon or off of dust in the solar system. Some of the light comes from the gas in the upper atmosphere. And a tiny bit is starlight from other stars.
Since we astronomers want to study light from specific stars or galaxies, we need to get rid of this glow. In the computer age, this is done by measuring the light from a star and measuring the light from the same size of spot in an area with no stars, and subtracting the two. And this generally works very well! Sometimes it works a little too well, and you can miss interesting things.
This happened to me recently. I was looking at stars, and one of them was a little strange. It appeared to have light coming from specific wavelengths (or exact colors) that most stars don't have. The extra light that I saw had the signature of hydrogen moving toward me at high speeds -- over 100 miles per second! There are plausible explanations for this, many of which are quite interesting. But still, something didn't seem right.
When I looked closer, I noticed that my sky subtraction didn't seem to be working right for that star, so I was seeing light from the sky, not from the star. But this is still odd -- the night sky doesn't glow with the light of hydrogen, and the sky is not moving toward the ground at 100 miles per second (unless the sky really was falling. When I looked at the sky in my data for the first time, I also saw other atoms that aren't in our atmosphere. In fact, the night sky looked a lot like a nebula such as the Cat's Eye Nebula. But in pictures of this patch of sky, I hadn't seen any nebula. What was going on?
After a few hours of research, I found that the patch of sky I was looking at was on the edge of the Cygnus Superbubble, a bubble of very hot gas that was made by several groups of hot stars and supernovae. The superbubble is huge -- 15 degrees across, or 30 times the size of the full moon. This is why I hadn't seen it on my image -- the light is spread over a very large area. My pictures were only 1/10 of a degree across, or over 100 times smaller than the size of the bubble.
While I was disappointed I hadn't discovered anything new, there is still science that I can learn from this. And I have learned to take a closer look at the blank sky, because I don't know what it might be hiding!