In 1929 Edwin Hubble showed (to an irritated Einstein) that distant galaxies were moving farther and farther away from the Earth, picking up speed the farther they traveled. It was the birth of Hubble's law, which says that the more distant the galaxy the greater its velocity or redshift.
Scientists concluded that for Hubble's observations to make sense, the universe must be expanding, swelling like a balloon or a loaf of bread in the oven. The beginnings of the Big Bang Theory began to emerge (hey all this expansion had to start from a single point, right?).
There are many hypotheses and much debate over how expansion occurs, but it is generally believed that expansion is the same everywhere, progressing uniformly. That may soon change. Recently, a team of American and Canadian researchers discovered that a certain region of our universe (400 million light years away to be exact, and thats considered 'close to home") is not expanding uniformly but rather unevenly; expansion is faster in one half of the region than the other.
What could be causing this boost of expansion? The researchers aren't sure yet, but if confirmed their results provide more evidence of obscure forces like dark energy and more recently, dark flow. Relatedly, scientists believe that dark energy is causing expansion as a whole to speed up.