What is dark matter? I mean it is not from school but the answer seems complicated, can someone answer this for me?

Answer 1

When you calculate how objects such as visible galaxies should move under the influence of the gravitational fields of known, observable, masses, the results do not match the motions. To make the computations come out right, mass is imagined that we can not see.
Look at the definition: https://www.merriam-webster.com/dictionary/dark%20matter

Answer 2

Here is a much more detailed description:
https://www.britannica.com/science/dark-matter