At the age of 13 you might find this hard to believe but scientists have hardly figured out anything. There are many things that are totally not known, and they are not that complex. Here are ten ordinary ones:
1. How smell works. There are lots of theories, but nobody really knows how our noses work. They have found the sensors in the nose, and they think there are about a thousand, but they don't know for sure exactly what the sensors do, how they detect things, what smell is, or how our brains take the results from our smell sensors and turn them into smells and memories of smells. All this is totally not known.
2. Why we need to sleep. You'd think by now we would know, and of course there are lots of theories, but scientists have no idea why people and animals sleep.
3. Why and how cats purr. Nobody knows.
4. Why sugar tastes sweet. This is similar to the smell thing. We don't know.
5. Gravity. Nobody knows what causes it, or how it is transmitted through space.
6. Aging. Nobody has figured out why animals age, and why some live way longer than others.
7. How cells know how to change themselves into other things. When any creature starts, including you, it starts as just one cell that divides billions of times until the whole animal is formed. Nobody knows how the different cells, like bone cells, skin cells, eye cells, stomach cells, decide to become what they are.
8. Dark matter and dark energy. When astronomers look into space, they can see a certain number of stars and you can work out how much matter (or stuff) this is. You can also work out how much energy they give out. However, if you consider all the gravity required to hold it all together, it turns out there must be tons of other stuff out there that we cannot see. This is called dark matter. Also, the same is true for energy. It turns out that what we can detect and see is only 5% of what is out there. That means after you count all the stars, planets, galaxies, dust clouds, and everything you see in space, you've still only accounted for 5% of the whole universe. So what is the rest? Nobody knows.
9. The genetic code. Although scientists have figured out the whole human genome, they have no idea what it does. If you examine it and figure out which parts of the code are instructions to make proteins (molecules of life), it turns out this accounts for only 3% of the whole code. So the question is: what is the other 97% for?
10. Finally, have you ever noticed when you are in a group of people and one person yawns, other people yawn right afterwards. Why is this? Nobody knows.
I've only scratched the surface. You will find that in any branch of science as soon as you start asking questions, it does not take very long for the scientist to say, "That's a very good question. We just don't know."