We know that the Universe contains every star and planet that we can see! We know that it is gigantic! Enormous! But we don't know for sure where it ends, if it ends at all! We don't even know for sure what shape the Universe is!
If we're in a room with ceilings that are eight feet off the ground, we can measure to find where the center of the room would be. From the center of the room, it would be four feet to the top (the ceiling) and four feet to the bottom (the floor). The room is a box shape and we can find all of the edges! Thinking about the Universe isn't like thinking about a room. We just don't know what shape the Universe is. Different scientists have different ideas of what the shape could be, but it might always be unknown!
The Universe might have any one of many shapes. It's difficult to imagine some of them because we are such a small part of the Universe, and we can't see the big picture. There are still some strange shapes that we can observe, right here on Earth! Can you imagine a piece of paper only having one side? You might think if there's a front, there has to be a back. Can you imagine paper with only a front? There's a strange shape called a "Mobius band". If you want to make one, you'll need a pencil, piece of tape or some glue, a piece of paper and a grown-up to cut a long thin strip of it for you""about an inch wide, as long as you can make it. Right now, the strip of paper has a front and back. On the front side put an "X" at each end. Now twist the paper into a loop, but twist it so that both Xs face each other. Tape or glue the paper into this shape. It should look like a regular loop but with a twist! Now, draw a line around the paper. You can start anywhere and follow it all the way around the loop. You'll end up back where you started, without having crossed any edges! That means your piece of paper only has one side! The Universe probably isn't shaped like a Mobius band, but there's a good chance it is a tricky and twisted shape that makes it tough for scientists to figure out what that shape is!