To hear them tell it, the biggest and most influential Silicon Valley companies don't make any distinction between women and men when payday comes around. Salesforce, Microsoft, Apple, Facebook, and Google are among the ones that say they have no gap.

The problem is, practically none of these companies offer any evidence to back up their words. And the notion of simply taking the industry at its word is a harder sell following allegations of extreme gender pay discrimination at Google trotted out on Friday by the U.S. Department of Labor.

"At this point the department has received compelling evidence of very significant discrimination against women in the most common positions at Google headquarters," Janet Herold, regional solicitor for the Department of Labor, told The Guardian.

Google has refuted the claims, but the company has yet to share its employee salary data, as it would have to do to prove its innocence. And that strikes some critics as cause for extra scrutiny.

"How can you get better if you can't show that you're getting better? And many of these companies don't show that," says Leslie Miley, executive in residence at Venture for America, an organization that trains entrepreneurs. "They want us to trust them."

It's not that Google has a hard-and-fast rule against sharing internal data. Every year Google publishes a report showing the diversity of its work force. And every year Google publishes a report showing government requests for user data.

Google's very mission is to "organize the world's information and make it universally accessible and useful." But when it comes to offering more than a blog post claiming to have pay parity -- when it comes to offering concrete data and information -- Google and its Silicon Valley peers seem to have drawn the line.

"A good compensation audit would factor in the skills, education, work experience, and/or performance of employees of different demographics and not just roles/titles as they do one-to-one comparisons of salary," says Stephanie Lampkin, founder of Blendoor, an app designed to help companies hire more women and people of color. Blendoor recently released an index that scores companies based on their diversity.

"I am certain there are women and underrepresented minorities with advanced degrees and five-plus years more experience who are paid less than white men simply because they have a different role/title," Lampkin said. "But these gender pay gap analyses don't reflect that."

Lampkin's hunch is supported by a recent analysis by Hired, a company used by most major companies in Silicon Valley to discover and hire workers. It found that on average women in tech are offered 4 percent less. Additionally, in 63 percent of situations, women receive lower offers than men for the same jobs at the same companies.

"When you look at the research, it shows that women get hired at lower levels than men," said Ellen Pao, Kapor Capital's chief diversity and inclusion officer.

The call for this data goes beyond doing social good. There are business reasons behind it. Though these companies do not share compensation, eventually employees tend to find out what their colleagues are making.

"When you find out that you're being paid less than your male colleagues, you are directly being told that you're valued less," says Alaina Percival, CEO of Women Who Code. "You're going to leave the company sooner, you're less loyal to the company, and you're going to be a less productive employee."

When you factor the money companies spend on marketing job openings, recruiting and interviewing candidates, training new employees, and lost productivity, the cost to replace employees far outweighs simply paying them their market value by thousands, if not tens of thousands, of dollars.

"It's fiscally irresponsible," Percival says.

Perhaps the Silicon Valley whales will take inspiration from a minnow in their midst. Buffer, which makes a social media content sharing tool, this month shared its data, showing exactly how much male employees earn ($95,221 on average) compared with female employees ($92,817). The report is not perfect -- it omits data based on ethnicity, and it has been determined that women of color earn far less than other demographics, according to Hired. Nonetheless, Buffer's forthrightness represents an implicit promise to professionals who might be considering working for the company.

"If a company is being up front about how they pay all employees, I think that is something that will attract candidates who have transparency and equality top of mind," says Hailley Griffis, a spokeswoman for the company. "We've seen this at Buffer."