If the curvature is fairly weak, Newton's laws of gravity can explain most of what is observed. For example, the regular motions of the planets. Very massive or dense objects generate much stronger gravity. The most compact objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip. Scientists today call such an object a black hole .
Most physicists believe they do, basing their views on a growing body of observations.
In fact, present theories of how the cosmos began rest in part on Einstein's work and predict the existence of both singularities and the black holes that contain them. Yet Einstein himself vigorously denied their reality, believing, as did most of his contemporaries, that black holes were a mere mathematical curiosity. They should be strong sources of gravitational waves . As such, black holes should reveal much about gravity, a fundamental force in the cosmos. Confirmation that they exist will strengthen confidence in current models of cosmic evolution, from the Big Bang to the present universe.