00:00There are more stars in the universe than you could ever count.
00:02So why is the night sky pitch black?
00:04Most people assume it's simple.
00:06Stars are just too far apart, with mostly empty space between them.
00:09But that can't be the whole answer.
00:11If the universe were infinite, eternal, and unchanging,
00:14almost every direction you looked should eventually end at a star,
00:17making the entire sky glow.
00:19But the universe hasn't existed forever.
00:21It has a finite age, so light from extremely distant regions
00:25hasn't had enough time to reach us.
00:27And the universe is expanding.
00:28As distant light travels through expanding space,
00:32its wavelengths get stretched,
00:34sometimes far beyond what human eyes can see.
00:36So the darkness between the stars isn't just empty space.
00:40It's evidence that the universe had a beginning,
00:42and that it's still expanding.
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