"Apparently, a great deal of dark, unseen material exists, whose gravitational pull is responsible for the motions of the stars and galaxies that we see."

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If we can avoid disaster for the next two centuries, our species should be safe as we spread into space. If we are the only intellegent beings in the galaxy we should make sure we survive and continue. . . . Our only chance of long-term survival is not to remain inward looking on planet Earth but to spread out into space. We have made remarkable progress in the last hundred years. But if we want to continue beyond the next hundred years, our future is in space.

— Stephen Hawking