Why is this the answer?

The answer is gravity because this incredible word is talking about the invisible force that holds our entire world together. It is a tricky riddle that makes you think about something you cannot see, but that is the strongest power on Earth. Let us look at the first big clue: "stands the test of time and holds fast to the center of everything." Gravity is always working. It has been pulling on the Earth since the very beginning of time, and it is the only reason why we are not all floating off into space. This powerful, amazing force comes from the center of the Earth and pulls you and everything around you right down to the ground, no matter where you are standing. That is how it holds fast to the center, like a giant, invisible magnet. Now for the next part: "everyone will try at least once in their life to move around this word, but in fact, unknowingly, they use it every moment of the day." Think about what happens when you jump high up in the air or launch a small toy airplane. When you do those things, you are trying to move away from gravity, but it always brings you back down to the ground. Even an astronaut in space is just orbiting Earth, still slightly caught in its pull. You use gravity all the time without even noticing it. It is what keeps your chair on the floor, your juice in your cup, and your feet firmly planted while you run and play outside. You rely on it for every step you take. The riddle is perfectly right when it says it "belongs to everyone, to all living things, but no one can master it." From the smallest ant to the biggest elephant, gravity affects us all, whether we are sleeping or wide awake. Even with incredible science, like flying a plane or launching a rocket into the sky, people can never truly turn gravity off. It is the greatest natural rule in the universe, and it is always there, pulling on you every single second.