New player need some help!

Hey guys im pretty new to the game! been leveling up with some friends, one thing i dont understand is why i keep playing against players like platinum and diamonds as a level 20, i dont understand how this match making system works, the people i play with are bronze and silver, so why is every team we play against gold and higher usually? i dont understand how this is balanced at all, for instance look at my last rift game, 4 players were gold and higher and one player was silver... i dont understand riot why does this happen?