You can see a clock in a spaceship moving past you at 90 percent of the speed of light. according to you, how much time would pass while the clock in the spaceship ticked through one minute?

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me

3. dark matter

empty space could also be a simple vacuum

just copy what u wrote and paste and search in google