They are the people who have bought new health insurance since the start of this year but have chosen for one reason or another to bypass the state and federal exchanges that opened last year under the Affordable Care Act. While the exact number is unknown, some health care experts estimate that it may be in the millions.

Politicians and policy makers have focused on the number of people who signed up through the exchanges — at nearly seven million and counting a day after the March 31 deadline — but they have largely overlooked the group that did not use the exchanges, even though it could have a major impact on the program’s financial success in the years ahead.

Because insurers place customers in the same risk pool, regardless of how they enrolled, “these lives count every bit as much as the ones that came in through the exchange,” said Gary Claxton, a vice president and health care expert at the Henry J. Kaiser Family Foundation.