Don’t capitalize black hole.
Add a comma after second. (If you want a more precise number for the speed of light in miles per second, it’s 186,282. The speed of light is usually expressed in meters per second, though, which is 300,000,000, or 3×10^8. Actually, the exact speed — as far as we know, because we can’t be certain until we get some precise measurements from outside the heliopause — is 299,792,458 meters per second. For most people, most of the time, rounding to 3×10^8 is just fine.)
Notice that the person who made this example didn’t capitalize black hole the second time? If you’re going to use nonstandard capitalizations, at least be consistent. (*mutters something about cards and dim lighting*)
Typo alert: There’s an extra space before the period after extreme.
The last sentence… I know what it’s trying to say, but that’s not what it does say. Word order matters, friends.
The gravity of a black hole is so strong that nothing, not even light, traveling at 186,000 miles (300,000 km) per second, can escape it.
Near a black hole, time becomes extremely slowed down. Time essentially stops. And Einstein’s theory predicts that at the center of a black hole, time is destroyed.