Poker has a gambling element to it, which leads some players to question if poker is just down to luck. Although there is a serious amount of luck involved in poker, the fact the same big names regularly win in cash games and tournaments shows that there is skill involved.
In general most reputable sites like pokerstars, 888, and many others cannot afford to be Rigged! they are monitored by Gaming commissions from many different Countries, and if one found anything that could be Crooked or Rigged, It would be their downfall and they would lose the trust from players all over the World.