MMR is raised by winning games. It's dropped by losing games.
Just for positivity's sake, I also want it to be known that SOME of the silvers you're getting matched with could either be DQing with a gold friend or they just have high MMR themselves.
Websites like op.gg and lolking say they can tell you your MMR, but since Riot doesn't release the formula for MMR the stats you get from sites like that aren't necessarily accurate. They generally use their own formula that takes the rank and recent wins/losses into account in determining MMR.