Considering matterand energy together the smallest component of the universe seems to be

Visible matter is a small fraction of the total mass-energy in the universe, Detectable electromagnetic (including microwave) radiation has more energy than visible matter.

Greater still is what is believed to be "dark" mater and "exotic" energy that cannot currently be detected as electromagnetic radiation.

See the pie graph at
http://universe-review.ca/R02-06-darkenergy.htm
for further details