Answer:
On a planet Behind the darkness
Explanation:
According to Mrs Which, Meg's father has been trapped behind the great darkness. Mrs Whatsit then give Meg the hope by telling her that they are traveling so as to go and help him. And that they would be going by tesseract. Meg does not understand this. The tesseract is then explained to be a shortcut between time and space. And it makes travel between such distances to be quicker as well as easier.
