Yes 3Mly across, equal to the distance from here to M31. Here is some more info:
http://news.bbc.co.uk/2/hi/science/nature/7587090.stm
"The latest astronomical observations suggest that dark matter makes up some 23% of the Universe. Ordinary matter - such as the galaxies, gas, stars and planets - makes up just 4%. The remaining 73% is made up of another mysterious quantity; dark energy, which is responsible for speeding up the expansion of the cosmos."