Final answer:
A black hole is an object in space with a gravitational field so strong that it can absorb light, which is why it is often referred to as the heavy part of space that absorbs light.
Step-by-step explanation:
Based on the provided text, the part of space so heavy that it absorbs light is a black hole. Black holes have gravitational fields so intense that nothing, not even light, can escape once it falls within their event horizon. This concept of a black hole originated from the idea that if an object had an escape velocity greater than the speed of light, then light could not escape it, an idea that was later confirmed by Einstein's theory of general relativity.