English: Astronomers have probed the deep sky with NASA's three Great Observatories for hidden black holes and come to the conclusion that most black holes cannot be seen in visible images. The image on the left from NASA's Hubble Space Telescope shows 1/200 of the full field of sky known as the Great Observatories Origins Deep Survey, or GOODS. It highlights three X-ray sources (circled) and many other galaxies. The image on the right is made up of data from Hubble and NASA's Spitzer Space Telescope and shows the same region. The two "hard" X-ray sources (sources detected only at the shortest X-ray wavelengths and indicated here with yellow circles) are very faint in the visible but much more luminous in the infrared. This data suggests that the X-ray sources are black holes hidden behind a screen of dust.