Professor Brian Cox continues his exploration of the universe with a journey into darkness. The centre of the galaxy is home to an invisible monster of unimaginable power - a supermassive black hole named Sagittarius A*. Weighing four million times the mass of the sun, it's an object with such an immense gravitational field that nothing can escape - not even light. For decades, black holes existed purely in the minds of theoretical physicists - the idea was so absurd, scientists thought they couldn't possibly exist in nature. But recent astronomical breakthroughs have confirmed not only that black holes like Sagittarius A* exist, but that these bizarre invisible objects may be the ultimate galactic protagonists