The average premium for a 27-year-old enrolling in the most popular silver plan fell 4 percent for 2020, according to the federal Centers for Medicare and Medicaid Services, which oversees HealthCare.gov. Some states are seeing double-digit premium declines, and more insurers are offering coverage on the marketplace, providing greater choice, the agency said.

Details vary, however, based on people’s income, the plan they choose and where they live, so some will see higher costs. Premiums may still be steep for some people who don’t qualify for the tax credits; people are ineligible if they earn more than 400 percent of the federal poverty level, or about $50,000 for a single person and $103,000 for a family of four.

While the Obamacare market has stabilized, many people remain in the dark about open enrollment. Just 5 percent of uninsured people are aware of the federal sign-up deadline, according to polling by Get America Covered, a nonprofit group that promotes health coverage.

“One of the biggest challenges is letting people know that this is the time to go and shop,” said Joshua Peck, a co-founder of the group and a former chief marketing officer of HealthCare.gov.

Through last Saturday, about 1.7 million people had chosen a plan on HealthCare.gov, down about 13 percent from the same period last year, according to numbers released by the government. (The Centers for Medicare and Medicaid Services said direct comparisons with prior years were difficult because many factors influenced enrollment. In addition, the agency said, this year’s measurement period contained one day fewer than last year’s.)

There’s typically a rush of enrollments before the deadline, so the trend could change. But Mr. Peck said it was worrisome that renewals, in particular, were down 16 percent from last year. People who don’t actively renew are automatically re-enrolled in the same health plan. But, he said, they often drop coverage in the first few months of the year because they find their plans have changed or they are getting less financial help because their income rose.

Part of the problem may be that the Trump administration has slashed funding for the marketing of HealthCare.gov as well as for independent “navigators” who help people choose plans and enroll, according to the Kaiser Family Foundation.