Why is this the answer?

The answer is shadow because this super clever riddle is talking all about the dark shape that follows you almost everywhere you go. It uses wonderful clues to describe this mystery that is actually made by light. Let us look at the first clue, "By Moon or by Sun, I shall be found." This is a very important hint that tells you exactly when you can see a shadow. Think about playing outside on a sunny day. When the strong light from the sun hits your body, your shape is blocked, and the area behind you that the light cannot reach becomes a shadow. Even when the sun sets, sometimes a big, bright moon acts like a giant, soft flashlight in the night sky. If you are outside during a full moon, you can still see your moon shadow stretching on the ground. So, whether it is the bright sun or the soft moonlight, both kinds of light help you find a shadow. Now, let us look at the second part of the riddle: "Yet I am undone, if there's no light around." The word "undone" means that the shadow completely disappears or is cancelled out. This is the key secret to the puzzle. If you walk into a closet and close the door so it is perfectly dark, you will notice that your shadow is gone. A shadow is not a real object you can touch, but it is just a space where light has been blocked. It needs light shining onto an object to be created. If you shine a lamp on your favorite toy, you see a shadow of the toy. But if you turn the lamp off, the shadow vanishes instantly. Since the shadow is found in the light of the sun and moon, but goes away in the complete dark, it perfectly matches all the clues in this fun riddle.