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. (10[1])An equation (10[1])of state of pressure (-5[1])equal to negative (10[1])density characterizes this substance, (10[2]-5[1])whose density remains constant (10[1])as space expands. (10[3]-5[2])An era dominated (10[1]-5[1])by this substance began roughly 4 billion (-5[1])years ago, (10[1]-5[5])when (-5[1])the (10[1])expansion (-5[2])of the (-5[1])universe (10[1]-5[1])began (-5[3])to accelerate. (10[3]-5[4])This (10[1])substance (10[2]-5[2])is (10[1])represented (-5[2])by a quantity (10[1])that Einstein (-5[2])called (10[1]-5[1])his “biggest (-5[2])blunder” (10[3]-5[6])when (-5[3])adding (-5[3])it (-5[2])to (-5[1])his field (10[1]-5[1])equations, (10[1]-5[3])known as the cosmological (10[1])constant. (-5[3])For 10 points, (-5[1])name this mysterious substance (10[1]-5[6])that accounts (-5[2])for (-5[1])over 70 (10[1]-5[1])percent (10[1]-5[4])of the universe, (10[3]0[1]-5[1])along (10[1])with (0[1])matter and (0[1])dark matter. (10[1])■END■ (10[39]0[18])