Dark Energy, Man.

The scientific breakthrough of the year:

Around 23% of the universe is made up of another substance [besides stars], called "dark matter". Nobody knows what this undetected stuff could be, but it massively outweighs all the atoms in all the stars in all the galaxies across the whole detectable range of space. The remaining 73% is the new discovery: dark energy. This bizarre force seems to be pushing the universe apart at an accelerating rate, when gravitational pull should be making it slow down or contract.

Now we know that the universe is about 13.7 billion years old. Science Daily has more on the story, with an additional list of significant scientific discoveries of 2003, including this sad and disturbing one: "Climate Change Impacts: Global warming was no longer an abstract concept in 2003. Scientists reported melting ice, droughts, decreased plant productivity, and altered plant and animal behavior."