Several people contributed to this amazing discovery. The first person to figure out that the Milky Way (the white band you see on a clear night going from horizon to horizon) was made of individual stars was Galileo Galilei - the same person who discovered the moons of Jupiter. Caroline Herschel and her brother William spent a lot of time mapping out the stars and then Annie Cannon introduced a way to measure and classify them. Nobody's exactly sure how many stars there are in our galaxy, the number's somewhere between 100 billion and 400 billion. This number is gathered as a bit of a guess. We look at one part of the sky, count the stars in it...then multiply by how much sky the milky way takes up! Edwin Hubble then discovered our galaxy wasn't alone, that there are other galaxies, billions and billions of them. So the credit for this discovery was very much a group effort from team Science!
When you squeeze a gas, you put it under greater pressure, the particles collide into each other and the walls of their container more, heating the whole system up. If you rapidly expand it, the opposite happens, the gas loses thermal energy. By repeatedly compressing and expanding a gas, we cause it to lose heat to its surroundings (the pressurised container). This is called the Joule-Thomson effect and when we do it with Nitrogen, it cools down so much that the particles stop moving around, their volume decreases, temperature drops and they enter a liquid state.
Dark matter is the as-yet unclassified source of extra gravity in the universe. Dark energy is the as-yet unclassified cause of the universe's accelerated expansion, faster than would be predicted by the Big Bang model. Dark matter is an extra source of gravity, dark energy is an unexplained way of gravity behaving. We don't really know that DM is a type of matter and we don't know if DE is actually a form of Energy. They're really placeholder names until we know more about them. While we can use E = mc^2 to equate normal matter with the energy of photons coming out of a nuclear fusion reaction (one of the equation's many uses) it isn't clear whether we can use this equation to make DM and DE equivalent to each other.