What is 'a mysterious force' given in the passage?

Directions: The passage is followed by five questions. Read the passage carefully and answer the question that follows. For the question four probable answers bearing options (1), (2), (3) and (4) are given. Only one out of those is correct. You have to choose correct answer.

The Hubble enables us to see farther than any telescope on Earth. It has helped us learn about how solar systems and galaxies form, as well as the existence of dark energy, a mysterious force that causes the universe to expand. Hubble, the powerful telescope, is the size of a school bus. It orbits Earth every 97 minutes, moving five miles per second. Since Hubble is beyond Earth’s atmosphere, it can take clearer pictures of outer space than any telescope on Earth.