Eliminating this substance is the goal of the DGP braneworld model. So-called “phantom” forms of this substance are an extreme case of hypothetical quintessence models. An equation of state of pressure equal to negative density characterizes this substance, (10[1])whose density remains constant as space expands. An era dominated by this substance began roughly 4 billion years ago, (-5[1])when the expansion of the universe (10[1])began to accelerate. (-5[1])This substance (10[1])is represented by a quantity (10[1])that Einstein (-5[1])called his “biggest blunder” (-5[2])when adding it to his field equations, known as the cosmological constant. For 10 points, name this mysterious substance (-5[2])that accounts for over 70 percent (10[1]-5[1])of the universe, along with (0[1])matter and dark matter. ■END■ (10[4]0[2])