Because Gliese 581 is only 1% as bright as the Sun, it exudes little warmth, and its habitable zone lies much closer. At this distance, planets get locked into strong gravitational pulls that tend to slow their rotation.
Trojan asteroids are objects that exist in gravitationally stable regions in front of or behind another body. The asteroid shares the same orbit with Earth, but the two will never collide.
Anything but empty space, a black hole is a great amount of matter packed into a very small area, resulting in a gravitational field so strong that nothing, not even light, can escape.
Researchers believe that the gravitational effect of dark matter makes galaxies spin faster than expected, and that its gravitational field deviates the light of objects behind it.