Mrs. Which reveals that Mr. Murry has been captured and is behind the dark shadow. And they are going to get to him by tessering. Mrs. Whatsit then explains to Meg what a tesseract is: it's like a wrinkle in time and space, where you simply bring two spots in space and time together and cross over from one to the other. When Charles and Calvin understand her explanation but Meg doesn't, Charles gives Meg a more mathematical explanation, likening the tesseract to the fifth dimension (with helpful diagrams included in the book). She understands a little more.
The author makes use of diagrams and drawings in the book to visually explain to the reader what a tesseract is. So visualizations are sometimes the best way to understand concepts…just not people.
Then the group once more tessers. This time, it's an even more painful experience, since the Mrs. W's accidentally take them to a two-dimensional planet (on which the children can't survive, as they're three-dimensional creatures), but a few seconds later they're whooshed off to another planet in Orion's belt. As they cross a nondescript plain and enter a dark stone cavern, Mrs. Whatsit explains that they're there to visit the Happy Medium. The Medium looks rather like a fortune-teller with a crystal ball, but she shows the children the planet Earth, which seems to have a "smoky haze" over it. This, as the children can guess, is the same Black Thing they saw covering the stars earlier.
How ironic that Meg meets the Happy Medium as a character, since both her mother and her brother have told her that she needs to find a happy medium to sort out her own feelings. The Happy Medium shows the children both bad things (Earth covered in the Darkness) and good things (a star fighting the Darkness), balancing out their perspective and ultimately making it a hopeful one.
When Calvin demands to know exactly what that dark shadow is, Mrs. Which tells him that it is Evil, the powers of darkness—and that they are going to continue to fight it. The battle against evil is being fought all over the universe, and some of the best fighters for the good have come from Earth. When Charles asks who, Mrs. Who smilingly quotes from the Gospel of John, "And the light shineth in darkness; and the darkness comprehended it not." "Jesus!" Charles Wallace exclaims. And not only Christ, they realize, but also all the great artists, composers, writers, saints, and scientists that have ever existed on earth: Michelangelo, Beethoven, Shakespeare, St. Francis, and Euclid, to name a few.
Evil is directly personified as the Black Thing. And similarly, there are concrete people who are the top fighters for the good: first and foremost, Jesus Christ. The author's Christian beliefs clearly come to the forefront here. But then other great non-Christian writers and thinkers of mankind are mentioned as fighters for the good, so the author is not limited to an only-Christian view of good in the world, and Jesus is equated with others who have been called geniuses but never gods.