PRACTICE TEST 2 What is the main topic of the talk?

[00:00.00]Listen to a talk in a physics class.
[00:01.00]Professor: Dark matter is a type of matter that doesn't emit, absorb, or reflect light, making it invisible to current instruments. Despite being unobservable directly, it's believed to make up about 27 percent of the universe's mass-energy content. How do scientists know dark matter exists? Well, all the stars in a galaxy rotate around the center of their galaxy. According to basic physics, the farther a star is from the center of a galaxy, the slower it should rotate. However, observations show that stars in the outer regions of galaxies rotate at unexpectedly high speeds. This anomaly suggests that a massive unseen matter is exerting gravitational force, keeping the stars moving faster than expected.
[00:02.00]Dark matter is hugely important for the structure of the universe. Computer simulations show that galaxies wouldn't have formed the way they did if dark matter hadn't been around. But what does dark matter consist of? Well, there are different theories, so let's consider a couple of them.