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Summary
At one time, many physicists resisted the notion of a black hole, a ghostly, self-sustaining gravitational field so intense that not even light can escape. Now, theorists have few alternatives to these creepy holes in the universe. Any alternative to a black hole must be some dark, dense material orb that's bigger than a black hole of the same mass. One hypothetical possibility is a gravastar: a thin material shell filled with dark energy, the mysterious space-stretching stuff that appears to be accelerating the expansion of the universe. Another possibility would be a boson star, which would consist of exotic, massive Higgs bosons or nearly massless hypothetical particles called axions. Theorists say the study of black hole alternatives would expand if observers saw evidence that the current description of black holes isn't correct.
