WASHINGTON - More than one third of US marriages begin with online dating, and those couples may be slightly happier than couples who meet through other means, a US study found. Online dating has ballooned into a billion-dollar industry and the Internet “may be altering the dynamics and outcome of marriage itself,” said the study by US researchers in the Proceedings of the National Academy of Sciences. The research is based on a nationally representative survey of 19,131 people who married between 2005 and 2012.