The motion of galaxy clusters in the distant Universe has just yielded the largest-scale test yet of the laws of gravity. Across scales that span hundreds of millions of light-years, gravity continues to behave the way Isaac Newton predicted in his universal law of gravitation . According to this law, every particle in the Universe exerts a gravitational force on other particles proportional to its mass, and inversely proportional to the square of the distance between the centers of mass of the