In this image taken in the 1970s and provided by the Carnegie Institution of Washington, Vera Rubin uses a measuring engine. Rubin, a pioneering astronomer who helped find powerful evidence of dark matter died Dec. 25. She was 88. (Carnegie Institution of Washington via Associated Press)

Vera Rubin didn't plan to be a pioneering female astronomer. When she was 10, lying awake at her home in Washington, memorizing the paths of the stars outside her window, “I didn't know a single astronomer, male or female,” she once said in an interview. “I didn't think that all astronomers were male, because I didn't know.”

But pioneer she did. Rubin's work fundamentally changed astronomy by confirming the existence of dark matter, the invisible stuff that makes up 27 percent of the universe.

The way she worked changed astronomy, too. The field may have been all male when she entered it 70 years ago. But by the time she died Sunday, at age 88, that was no longer true. That's in large part thanks to Rubin: a brilliant mentor and fierce advocate for women in science.

“She made science kinder & the culture of research more human,” tweeted Mika McKinnon, a science writer and geophysicist.

Vera Rubin died Dec. 25, 2016. In this archive video, Rubin delivers the 2011 commencement speech at American University in Washington. She told graduates what she expects from the universe in the year 3000. (American University)

Those words hardly described science in the 1940s and 50s, when Rubin was at the beginning of her career. In an interview published by the American Institute of Physics, she recalled the day she told her high school physics teacher that she'd been awarded a scholarship to Vassar.

“He said to me, 'As long as you stay away from science, you should do okay,'" Rubin recounted. “It takes an enormous self-esteem to listen to things like that and not be demolished.”

But Rubin learned a lesson from the experience: “Rather than teaching little girls physics, you have to teach them that they can learn anything they want to,” she said.

Attitudes did not change as Rubin ascended in her field. Though she had degrees from Vassar and Cornell and had presented before the American Astronomical Society at age 22, she was barred from getting a PhD at Princeton because the university's graduate program didn't accept women. A few years later, she asked to attend a talk at the Applied Physics Lab, where her husband worked. She was rebuffed because “wives were not allowed” — never mind that Rubin was a scientist in her own right, working on her PhD in astronomy at nearby Georgetown University.

But if the institutions of astronomy would not accommodate her, Rubin was not afraid to change them. In the mid-1960s, she was granted access to San Diego's prestigious Palomar Observatory, an old boys' club so infamous astronomers called it “the monastery.” Though she could use the telescope, Rubin was informed that there was nowhere for her to relieve herself — the facility had no 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,” former colleague Neta Bahcall of Princeton University told Astronomy magazine in the fall. “She said, ‘There you go; now you have a ladies’ room.’ That’s the type of person Vera is.”

Speaking to CNN, Bahcall also recalled a time when an astronomer at the University of Chicago wrote to Rubin, asking her to advertise his school's program. She wrote back saying that if she were a promising young scientist just starting out in the field, she would never apply there because the program's faculty included no women.

Everywhere she went, Rubin fought for more women to be included. In 1976, when she found out that the first-ever Smithsonian Air and Space Museum planetarium show on the history of American astronomy would feature only men — all but one of them white — she lobbied for months for women to be included (her efforts were unsuccessful). She was part of a cadre of scientists and scholars who pressured Washington's exclusive Cosmos Club to admit women. She criticized the National Academy of Sciences for its dearth of female members. She met with politicians to discuss the need to create more opportunities for girls.

“All of us, men and women alike, need permission to enter and continue in the world of science,” Rubin wrote in a 1986 editorial in the journal Science. “… While such permission has generally been granted to bright men, it had always been less readily granted to young women and continues to be denied to many women even today.”

She described how an aspiring scientist might be the only female student in her department, endure more scrutiny than her male colleagues and be discouraged from seeking a high-profile position in her field. “This kind of gatekeeping also serves to limit opportunities,” she wrote.

Having made it past astronomy's gatekeepers, Rubin was devoted to keeping the way open for women behind her. “It is well known,” she once said, “that I am available 24 hours a day to women astronomers.”

This was not an exaggeration, according to Sandra Faber, an astronomy professor emerita at the University of California at Santa Cruz who once spent a summer working with Rubin.

“She was the first woman I encountered at that level,” Faber told the Los Angeles Times. “She helped me along at various crucial stages during my career. But each young person thinks they had the special relationship with Vera. She had that knack.”

Speaking to Astronomy magazine, Bahcall remembered how Rubin encouraged her younger colleagues.

“If they didn’t get a job or they didn’t get a paper published, she would cheer people up,” she said. “She kept telling her story about how there are ups and downs and you stick with it and keep doing what you love doing.”

Rubin never received the Nobel Prize for her work — a snub that many interpret as a sign of the Nobel Committee's gender bias (only two women have won the award for physics, and none in the past 50 years). Writing in Astronomy magazine, Sarah Scoles suggested maybe Rubin was like the dark matter she helped explain: invisible to the committee.

But, also like dark matter, she made her influence felt.