This may come as a bit of a surprise to the Young Earth Creationists out there, but we have a pretty good handle on the age of the universe, and it's not 6,000 years old. Nope. It's 13.73 billion years old, give or take 120 million years.
It's the 120 million number that amazes me - we know the age of the universe to fantastic accuracy. What's more, we also know the age of the Universe when recombination occurred to within 3,100 years ( Recombination happened when electrons were first able to be captured by protons, forming neutral hydrogen for the first time after the big bang) just 375,938 years after the big bang.
How do we know this much cool stuff? It's all in the five year report of NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), measuring the Cosmic Microwave Background - the photons that are left over from the big bang. Phil Plait has an excellent overview of the process:
It’s those photons WMAP sees. After 13.7 billion years, the expansion of the Universe has cooled the light, stretched its wavelength from ultraviolet to microwave. Another way to think about it is that the temperature associated with each photon went from thousands of Kelvins down to just a few, less than 3, in fact. That’s -270 Celsius, and -454 Fahrenheit.
That light emitted just after recombination tells us a vast amount about the Universe at that time. By carefully mapping the exact wavelength of the light and the direction from where it came, we can tell the density and temperature of the matter at that time. Incredibly we can also tell how much dark energy there was, and even the geometry of the Universe: whether it is flat, open, or closed.
Truly incredible. The important thing is that these results agree with the theory to an amazing degree - we can have high confidence that our theories of the beginning of the universe are not too far off the mark. That's what science is all about - generating testable theories that are either supported or knocked down by direct observational evidence. It's a process that has brought us incredibly far in our understanding of nature, and continues to build our knowledge.
Testable: what science is, and what religion isn't.