Astronomical measurements reveal that about 90% of the mass of the universe is invisible to us. This invisible matter, also known as dark matter, is likely to be “exotic” matter—very different from the elements that make up the periodic table (see Section 28.4). We know the dark matter is there because of its gravitational effects, but scientists can only guess as to its nature. What do you think dark matter might be made of? How soon might we know the answer?