I'm not an economist, nor am I great at analyzing data, so understandably I could be wrong about this. Based on the information on this one site though, DC has actually been gaining on Marvel as apposed to the other way around.
In 2003 Marvel sold 39.68% of the market in units sold, apposed to DC who was 30.60% (9.08% different)
In 2013 Marvel sold 36.97% of the market in units sold, apposed to DC who was 33.35% (3.62% different)
http://www.comichron.com/monthlycomicssales/2013.html
http://www.comichron.com/monthlycomicssales/2003.html
In dollars they have stayed relatively similar...
In 2003 Marvel sold 33.24% share of the overall dollars, apposed to DC who was 29.89% (3.35% different)
In 2013 Marvel sold 33.50% share of the overall dollars, apposed to DC who was 30.33% (3.17% different)
If you go back to the 90s, DC was even a smaller share of the comic market, as Image Comics held a much better percentage. Now though, DC is gaining, though understandably not everyone will be impressed with the product.
I may be interpreting that data incorrectly, which if I am, please be kind as I wasn't saying I was positive. The website might not be the greatest source of information either, I don't know. The information I found was in the margin along the left hand side of the screen.
-Also I guess this info only includes North American comic shops.