What theoretical stellar remnant is a cold, non-radiating lump of degenerate matter?
Answer
Black Dwarf
The theoretical endpoint for a White Dwarf, which is already a hot, dense stellar ember radiating away its stored heat, is the Black Dwarf. Over unimaginable timescales, billions of years in the future, this object will cool down completely until it no longer emits significant radiation. It will become a cold, dark mass composed of degenerate matter, stable because the electron degeneracy pressure still prevents complete gravitational collapse. Since the universe has not existed long enough for even the oldest white dwarfs to fully complete this cooling process, it is currently believed that no true Black Dwarfs physically exist yet in the cosmos.

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