What is a room you cannot enter?

The room in the Department of Mysteries that no one can go into is full of the force of love. Although Dumbledore never actualy says those Words, he alludes to it in chapter thirty-seven of "Harry Potter and the Order of the Pheonix".

"It contains a force that is at once more wonderful and more terrible than death, than human intelligence, than forces of nature... that power also saved you from possession by Voldemort, because he could not bear to reside in a body so full of the force he detests." (JK Rowling, "Harry Potter and the Order of the Phoenix", pgs. 843-844).

Dumbledore has previously said that Voldemort cannot stand love, so it follows to reason that the room is full of it.