One of the biggest puzzles of modern cosmology is the accelerated expansion of the Universe, which was observed at the beginning of the 21st century. One of the possibilities, how to obtain a model, is to add the cosmological constant into the Einstein equations. Second possibility is to change the theory of gravity. There are many ways, how to do it. We were investigating so called f(R)-theories. General Relativity is one example of these theories for f(R) = R. Here is my talk.