With 200 billion stars in our own galaxy, you might expect the night sky to be lit up like daylight. If you live in a big city, it just might be—but not because of starlight. But just why is it so dark at night? It took astronomers a very long time to figure it out.
![Hubble image of Galaxy NGC 4414 Hubble image of Galaxy NGC 4414](http://spaceplace.nasa.gov/news-images/ngc4414-hst.jpg)