Mice are merely the protrusion into our dimension of hyper-intelligent pan-dimensional beings who, unbeknownst to the human race, are the most intelligent species on the planet Earth. They spent a lot of their time in laboratories running complex experiments on humans. They paid Magrathea for the planet and will now collaborate to create a new one due to the interruption of Vogons.
At the outset, they were so fed up with the constant bickering about the meaning of life, which used to interrupt their favorite game, Brockian Ultra Cricket, that they decided to sit down and solve their problems once and for all.
They were the creators of Deep Thought, a stupendous super computer the size of a small city, to tell them the Answer to Life, The Universe and Everything. When seven and a half million years later it was realized they didn't know the question to the answer they'd been given, a second computer, of such infinite and subtle complexity that life itself would form part of its operational matrix, was created to work out the Ultimate Question. That computer was known as the Earth.
Some notable mice in the series are Frankie and Benjy.