Horizonlooksatdarkenergy-themysteriousforcethatisunexpectedlycausingtheuniverse'sexpansiontospeedup.Theeffectsofdarkenergywerediscoveredin1998butphysicistsstilldon'tknowwhatitis.Worse,itsveryexistencecallsintoquestionAlbertEinstein'sgeneraltheoryofrelativity-thecornerstoneofmodernphysics.Thehuntfortheidentityofdarken...