This thing is one of the four fundamental forces of nature, along with electromagnetism, strong nuclear force and weak nuclear force.
It's what keeps us from floating away into space, and what makes apples fall from trees.
But gravity is also weird as hell, and we don't really understand how it works.
You see, gravity is not just a force, but also a property of space-time.
Space-time is the fabric of the universe, and it can bend, stretch and warp depending on the presence of matter and energy.
The more mass or energy something has, the more it curves space-time around it, and the stronger the gravitational pull it exerts.
But here's the kicker: space-time is not only affected by matter and energy, but also affects them back.
Matter and energy can move through space-time, but they also follow its curvature.
This means that gravity can bend light, slow down time, and even create black holes.
Sounds cool, right? But there's a problem.
Space-time is not behaving the way we expect it to.
According to our best theories of physics, space-time should be contracting due to gravity, making the universe smaller and denser over time.
But instead, we observe that the universe is expanding faster and faster, as if something is pushing it apart.
What is this mysterious force that counteracts gravity?
We don't know.
We call it dark energy, but we have no clue what it is or where it comes from.
Some scientists think it's an inherent property of space itself, that has a negative pressure that drives space apart.
Others think it's a new form of energy that we haven't discovered yet.
And some think it's a sign that our theories of gravity are wrong, and we need a new one that can explain both the small-scale quantum world and the large-scale cosmic world.
Dark energy is not the only mystery of gravity. We also don't know what causes gravity in the first place.
Most physical theories say that gravity should be carried by a hypothetical massless particle called a graviton.
But we have never detected any gravitons, and we don't know if we ever will. Gravitons are supposed to be very weakly interacting with matter, so they are very hard to catch.
Another mystery is why gravity is so much weaker than the other forces of nature.
Gravity is about 10^40 times weaker than electromagnetism, which means that a tiny magnet can overcome the gravitational pull of the entire Earth.
How can this be?
Some physicists think that gravity might leak into extra dimensions that we can't see or access, making it appear weaker in our four-dimensional world
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