Components of the Universe

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Diverse lines of evidence lead us to believe that less than 5 percent of the universe is composed of the familiar particles of the Standard Model, and only 27 percent of the universe is composed of matter. The remainder is dark energy, which is the name we give to whatever is causing the expansion rate of the universe to accelerate, and also to whatever accounts for the discrepancy between the amount of matter in the universe and its geometry. Dark energy is one of the most important and intriguing phenomena in physics today. Is it Einstein's cosmological constant? Are we misunderstanding how gravity works on large scales? We can use quantum mechanics to calculate how much energy empty space should have—why is this number so wildly different from what we observe? Our ignorance leaves a great deal of room for future discovery. (Unit: 11)