Expelled from his life, Jason Dessen, a physicist, or Physics teacher? finds himself traveling through alternate realities, bringing to fruition the many outcomes (multiverse) theories.
The other realities have versions of him, and worlds that range from livable, inhospitable, to alien.
Dark Matter is a book about possibilities--what if I had taken path A rather than B or C.
As it turns out, another version of Jason, the one who took the prestigious 'career path,' invents the multiverse cube that leads to his current displacement.
The added element of an unknown psychotropic drug makes Dark Matter feel like a journey through the mind, which is intentional, perhaps. Jason gets to explore all the decisions he could possibly make, and where they'd take him.
When faced with alternate choices, testing and feeling out each universe, he's guided back to his own reality by the love of his wife and son.
Strangely enough, the New York Times review of Dark Matter suggests Crouch takes great care to keep the reader and the protagonist on the same page.
"...and dictates that whenever one gets too far ahead of the other, she or he will be blindsided by an unexpected revelation or an unobserved detail."
Imagine that--jumping ahead has consequences and the unknown is well, unknown.