I have a 60D too. Simply put, cameras focus best when the contrast is high. Note that this is different from "cameras focus best when it is bright". I am not sure how dark it was for you, but usually there is something you could do to focus in low light:
- focus on a light source that is of similar distance of your subject, like a light bulb or a candle
- manual focus
- add light to the scene so it can focus
Let's talk more about the third option, especially if it is almost too dark to even see with your naked eye.
You have a few choices:
- some constant light source, for example a torch or infra red pointer as suggested above
- using a dedicated external flash unit (that has infra-red light to assist focusing)
- use the built-in popup flash (so it flashes while trying to focus)
The first option is very unreliable. You have to aim it right, and unless you have 3 arms its not the easiest to aim and operate your camera at the same time. Also, in some places, the use of such light may not be allowed.
External flash units, usually, come with an infra-red focus assisting lamp. it is very dim so it causes the least disturbance, it is very fast and very reliable. I have a 580 EXII myself and the infra-red beam will also work with the 9 focusing points of my 60D. I have used this method and I was able to focus in pitch black. The drawback is the cost and weight of the flash unit.
The third option has the advantage that it is always with you. By default, it is turned on. However I have chosen to turn it off in the custom function, because it causes too much disturbance by emitting a pulses of short flashes. If I was shooting people, the pulsing of flash is hard on their eyes. It is also very noticable since its really bright. The chances of it working, from my experience, is fifty fifty. It is definitely slow, so slow that sometimes I will need to do two or three series of flashes to be able to get a focus. However, this costs nothing.
If the situation allows, and you have the time to retake the shot in case the focusing fails, you can use this method. However, if you can still see with your eye through the viewfinder, and if speed is an issue, I would recommend manual focusing. At least I can rely on my naked eye and get one solid chance of getting the shot, instead of firing pulses of flashes and have an 80% chance of completely missing the shot.
It is not easy to focus in the dark, but if it is not too dark, you can usually find something bright to focus on. Getting a dedicated flash unit is still the best solution. Moreover, if used skillfully, a flash can produce amazing results in low light situations.
The recent Canon 320EX flash is quite cheap and it comes with a constant LED light for videography. Although without an infra-red focus assist light so it works differently, it is still a very affordable solution to low light focusing and low light photography in general.
I hope this answer helps.