Vera Rubin, an astronomer who proved the existence of dark matter, one of the fundamental principles in the study of the universe, but who battled sex discrimination throughout her career, died Dec. 25 at an assisted living facility in Princeton, N.J. She was 88.

She had dementia, said a son, Allan Rubin.

Dr. Rubin’s groundbreaking discoveries, made primarily with physicist W. Kent Ford, have revolutionized the way scientists observe, measure and understand the universe.

The concept of “dark matter,” an unknown substance among stars in distant galaxies, had existed since the 1930s, but it was not proved until Dr. Rubin’s studies with Ford in the 1970s. It is considered one of the most significant and fundamental advances in astronomy during the 20th century.

[VIDEO: Astronomer Vera Rubin tells 2011 graduates her predictions about the year 3000]

Vera Rubin in her office at Washington’s Carnegie Institution in 2010. (Linda Davidson/The Washington Post)

“The existence of dark matter has utterly revolutionized our concept of the universe and our entire field,” University of Washington astronomer Emily Levesque told Astronomy magazine this year. “The ongoing effort to understand the role of dark matter has basically spawned entire subfields within astrophysics and particle physics.”

Dr. Rubin, who spent most of her career at the Carnegie Institution of Washington, continued to make new discoveries — including of previously unknown galaxies — into the 21st century. For years she was considered a leading contender for the Nobel Prize, but the award never came. Many attributed the oversight to gender bias among male scientists and prize committees.

The last woman to win a Nobel Prize in physics was Maria Goeppert Mayer, who shared the 1963 prize for her work on atomic structure. The only other woman to win a Nobel in physics was Marie Curie in 1903.

“Alfred Nobel’s will describes the physics prize as recognizing ‘the most important discovery’ within the field of physics,” Levesque told Astronomy magazine. “If dark matter doesn’t fit that description, I don’t know what does.”

As a girl growing up in Washington, Dr. Rubin built a rudimentary telescope out of a cardboard tube.

Early in her career, she made discoveries that challenged accepted theories in astronomy, but she was seldom taken seriously by other astronomers, most of them male. When she applied to graduate school at Princeton University in the late 1940s, she was flatly told, “Princeton does not accept women.”

Dr. Rubin forged ahead, ultimately receiving a doctorate from Georgetown University while raising four children.

Vera Rubin in 2010. (Linda Davidson/The Washington Post)

“I worked for almost all of my early career as a part-time person so that I could be home at 3 o’clock, and that was after they were all in school,” Dr. Rubin told Discover magazine in 2002. “It was almost overwhelming. I did a lot of my work at home.”

She struggled to gain admittance to leading observatories. In 1964, she became the first woman to receive formal approval to use the Palomar Observatory in Southern California.

When she arrived, she discovered that it did not have a women’s restroom.

“She went to her room, she cut up paper into a skirt image, and she stuck it on the little person image on the door of the bathroom,” astrophysicist Neta Bahcall told Astronomy magazine. “She said, ‘There you go; now you have a ladies’ room.’ ”

While fighting these battles on Earth, Dr. Rubin peered into the cosmos and examined the rotation of more than 200 galaxies. Among other findings, she determined that stars orbiting on the outer edges of galaxies moved at the same speed as those near the interior.

The discovery defied the accepted norms of astronomy, which held that the far-flung stars should move more slowly. To account for the uniform speeds, Dr. Rubin concluded that the distant regions of galaxies contained considerable amounts of a dense, unseen mass, or dark matter, which affected everything from gravitational pull to the shape of galaxies to how stars move in relation to one another.

Dark matter has not been directly observed, and its precise composition remains unknown, but scientists think it constitutes about 84 percent of the cosmos.

“So important is this dark matter to our understanding of the size, shape, and ultimate fate of the universe,” Dr. Rubin wrote in Scientific American in 1998, “that the search for it will very likely dominate astronomy for the next few decades.”

Vera Florence Cooper was born July 23, 1928, in Philadelphia. She moved with her family to Washington when she was 10. Her father was an electrical engineer.

She graduated from Coolidge High School in the District and, in 1948, from Vassar College in Poughkeepsie, N.Y. She received a master’s degree in 1951 from Cornell University and a doctorate in astronomy from Georgetown in 1954.

She taught at Montgomery College and later at Georgetown before joining the Carnegie Institution, a Washington-based research center, in 1965.

Dr. Rubin published more than 100 scientific papers and was on the editorial boards of professional journals and Science magazine. She published a collection of essays, “Bright Galaxies, Dark Matters,” in 1997. She was a member of the National Academy of Sciences and in 1993 received the nation’s highest scientific award, the National Medal of Science.

Her husband of 59 years, Robert Rubin, a physical chemist, died in 2008. All four of their children received doctorates in science or mathematics. A daughter, astronomer Judy Young, died in 2014.

Survivors include three sons, David Rubin, a geologist, of Santa Cruz, Calif., Karl Rubin, a mathematician, of Irvine, Calif., and Allan Rubin, a geologist, of Princeton; a sister, Ruth Burg of Washington; five grandchildren; and a great-granddaughter.

Dr. Rubin was active in promoting the careers of women in astronomy and other sciences and worked to help other women in astronomy and other sciences. But if she was disappointed at not receiving the Nobel Prize, she never said so in public.

“I think the question is,” she told The Washington Post in 2005, “are there women and have there been women who want to do science and could be doing great science, but they never really got the opportunity?”