Existing evidence suggests that the universe was once much denser and hotter than it is today. As it cooled and continued to expand, galaxies and stars formed within a few billion years after the Big Bang. It continues to expand today, and may do so indefinitely if the gravity of all the stars and matter in our universe is not enough to halt the expansion at some distant future date, tens of billions of years from now. Astronomical research hopes to determine whether the universe will expand forever or not, by weighing the universe gram by gram.