On a brisk morning in April, dozens of investors filed into a Marriott hotel conference hall in Kendall Square, the biotech hub of Cambridge, Mass., for the first Moderna Therapeutics platform science day. They gathered hoping to get a rare, inside look at the science behind what the start-up had long claimed to be a disruptive drug platform: carefully designed molecules called messenger RNA that prompt the body to make its own medicine. The concept has attracted billions in funding, but the company had largely kept details of how the technique works under wraps.