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, whose density remains constant (10[1])as space expands. An era dominated by this substance began roughly 4 billion years ago, when the expansion (-5[1])of the universe began to accelerate. This substance is represented (-5[1])by a quantity that Einstein called his “biggest blunder” when adding it to (-5[1])his field equations, known as the cosmological constant. For 10 points, name this mysterious substance that accounts for over 70 percent of the universe, along with matter and dark matter. ■END■ (10[2]0[1])