Fermions (the particles that make up matter, i.e. quarks and electrons) have no volume. Thus, one could say that matter takes up no space whatsoever. If you and I are literally nothing, depression is much more understandable.
Matter and energy are the same thing, but converting between the two requires effort.
Matter and antimatter collisions produce vast emissions of pure energy. The collision of a graviton and an antigraviton has unknown effects, but they are probably very energetic. Of course, neither particle is know to exist, so we can only speculate about their properties.
It is unknown whether space itself has a density. It obviously has a volume, and this volume can be altered by the presence of mass or energy. If it does have a density, it must also have a mass. This could explain why gravitational fields are able to affect it.
The effects of gravity are well understood, but we have no idea what it actually is.
Luminous matter is thought to make up only 4% of the observable mass in the universe. The rest of it is a mystery.
Black holes radiate energy, because otherwise they would reverse entropy, which simply isn't allowed. Let me explain. If you throw a bunch of diffuse gas intoa balck hole, it will become compacted by gravity, heating up. This would violate entropy. Thus, for the total entropy of the universe to keep increasing, black holes must radiate some energy over time.
"Only two things are infinite: the universe and human stupidity, and I'm not certain about the former" -Albert Einstein.
"Not only does God play dice, but he sometimes throws them where we can't see them" -Stephen Hawking. One would think God has better things to do, but who am I to judge.
Time flows more slowly the closer you are to massive objects. This causes the clocks in GPS satellites, which occupy a high orbit, to run faster than ones on the ground. In theory, a person wearing a watch would show a different time than a monkey wearing an identical watch, but the difference would be so tiny we can probably never measure it. Now, if you could live on a neutron star, you would age slower than someone living on Earth.
Two identical shells fired from two identical cannons will not land in identical spots, because of the effects of the atmosphere and the Earth's rotation.
Assuming you could make perfectly frictionless parts, you still could not make a perpetual motion machine, due to tiny variations in the Earth's gravitational field.
If you could travel faster than light, you would move backwards in time. However, this would reverse entropy and violate Heisenberg's Uncertainly Principle, which seems implausible.
Since mathematical proofs can only be defined in terms of mathematics, you must assume certain things are true without being able to prove it. Thus, math (and indeed all knowledge) is inherently flawed and unprovable. This forms the basis of Godel's incompleteness theorem.
A system which has perfectly chaotic behavior can be considered stable, because you can predict that no prediction can be accurate.