What scares Meg as they travel away from Uriel?

Before they
travel away from Uriel, Meg, Calvin, and Charles Wallace experience the Dark thing. It appears
as a shadow that blots out the stars. Meg is frightened:

What could there be about a shadow that was so terrible that she knew that there had
never been before or ever would be again, anything that would chill her with a fear that was
beyond shuddering, beyond crying or screaming, beyond the possibility of comfort?

They also see through the Happy Medium that the Dark thing is
partially covering the Earth. The Dark thing is the force of Evil in the universe. The children
are introduced to it on Uriel so that they can understand what they will have to fight against
on Camazotz, a planet completely overtaken by the forces of evil.

Although
Meg and the other children are frightened of the Dark thing, they are also reassured by Mrs.
Whatsit, Mrs. Which, and Mrs. Who that a force of good in the universe is battling the Dark
thing. The three older women are on the side of good, and Meg learns that on earth figures such
as Jesus, as well as famous artists and scientists, have fought the Dark
thing.