The Stanford CS178 lectures have a slide deck on Noise and ISO, which I shall attempt to paraphrase. There are several sources of noise, but the 3 biggest causes of noise in this photo are likely:
Photon shot noise
Photons arrive probabilistically following a Poisson distribution. This means is there is a
sqrt(intensity) variance in the recorded intensity of a light source. For darker images,
sqrt(I) is very close to
I, which causes significant noise. You can't really avoid this problem except by taking pictures of brighter things
Electrons in your camera's CMOS sensor are dislodged by thermal activity. Unless you live in a bucket of liquid nitrogen, it is too hot for you to avoid this type of noise. Going into the cold outdoors would reduce it. User Edd also points out that your camera has a Long Exposure Noise Reduction function, under C.Fn-3, which uses dark field subtraction in the camera to average out a lot of the thermal noise.
Heat causes the circuits that read out the data from your image sensor to record more noise. (This effect is reduced in CCD cameras.) Since you have a Canon, which uses CMOS, go outside into the cold.