The enthralling “Night Sky,” running off-Broadway in Manhattan, may be the accomplished Susan Yankowitz’s best play yet. Her first-hand knowledge of aphasia and exemplary research into astronomy are breathtaking as she embraces an insight of Stephen Hawking’s that the two abiding unsolved mysteries are the brain and the cosmos. She makes a poetic and dramatic case for the resemblance or correspondence between the black holes of the universe and the dark recesses of the human brain, and unponderously enlightens us in her serious and humorous, wise and profoudly moving play.