It was July 2013 and George Zimmerman was on trial for murder in the killing of Trayvon Martin, a 17-year-old Black boy who had been walking in his father's Florida neighborhood. The night the verdict was set to be announced, Alicia Garza and some friends gathered for drinks.

That night Garza woke up in the middle of the night crying. So she began writing out how she felt, in an emotional Facebook post. She wrote that she continued to be surprised "at how little Black lives matter."

"That's really what Black Lives Matter was for me," Garza told MSNBC. "And that's why I said Black people, I love you. I love us. And that our lives matter. And that we matter. And that Black lives matter."

The phrase "Black lives matter" inspired Garza's friend, Patrisse Cullors, to create a hashtag. Almost instantly, it went viral.

"I wanted it to go viral," Cullors, who co-founded Black Lives Matter with Garza and Opal Tometi, told CNN.

"On July 15th, 2013 I said that Alicia and I had created a thing called #BlackLivesMatter and we hope that it will be bigger than we can ever imagine," Cullors said. "Over the last seven years we've developed more infrastructure and we've become more organized."

Momentum around the movement has grown amid calls for justice following the deaths of Ahmaud Arbery, Breonna Taylor and George Floyd.

The phrase "Black Lives Matter" is being used now as more than just a hashtag -- it's a rallying cry. At protests, it's what many demonstrators write on their signs or chant as they march. Online, it's what many have used to spread the message against police brutality. And globally, it's being used by many to show solidarity with Black people.

While many believe the movement is more accepted than it once was, others remain skeptical of what it accomplishes and question its impact.

Here's what you need to know about the evolution of the movement, and why it matters.

Growth of a movement

Black Lives Matter is one of the most well-known organizations fighting for the well-being of Black people.

"We live in a country built to keep us away from these resources that we need," said Kailee Scales, managing director of the Black Lives Matter Global Network Foundation.

"Folks in the movement have been consistently fighting to reverse that trend, to raise awareness that this is not the way we're supposed to live," Scales told CNN.

Since the hashtag launched, the organization has become more formalized, taken on specific branding and branched out into nationwide chapters -- all in an attempt to solidify the group and allow them to create national campaigns while engaging the broader community.

Organizers put together a website, which led to the development of local chapters of BLM, first in Los Angeles in 2013 and then throughout the country, according to Scales.

After the protests in Ferguson, Missouri, against police brutality in 2014, the organization became more well known. In 2017 it became incorporated as the Black Lives Matter Global Network Foundation, Inc.

Today the network consists of 13 official chapters in the US, plus three more in Canada, according to the BLM website. Local chapters are semi-autonomous, meaning they "do the work that makes sense" for their communities, Scales said.

The structure allows the group to have an impact at all levels -- doing work both nationally and locally, she said. Each chapter works a little differently and has different focuses, depending on community needs.

The Chicago chapter, for example, focuses more on police accountability, while the Boston chapter does a lot of activity around mutual aid, Scales said.

The group has called for defunding the police for at least five years, Black Lives Matter Los Angeles co-founder Melina Abdullah told CNN. The most recent push began in May, and at the time the chapter had little support from local politicians.

After George Floyd's killing, however, the group gained momentum and received some support from a few members of the Los Angeles City Council.

And there is more work to be done, Abdullah said.

"(We) never thought that that (amount) was anywhere near enough," she told CNN. "But it's a step forward that we forced."

The different chapters are bound by a set of guiding principles, which work to ground the individual entities.

But they also work together. Chapters tend to work across state lines, Abdullah told CNN. When the BLM chapter in South Bend, Indiana, was getting started, Abdullah said the group worked closely with BLMLA -- specifically comparing notes on the terms of mayors Pete Buttigieg and Eric Garcetti.

It's worth noting that not every protest or demonstration is part of the Black Lives Matter network, as some may be organized by people simply using the movement's name. And not every person attending a BLM protest is a part of their local BLM chapter.

The movement is largely decentralized, with no one besides managing director Scales receiving pay, said Abdullah, who also works as a professor at California State University in Los Angeles.

Chapters tend to have a loose leadership structure, with a focus on on-the-ground organizing -- allowing chapters the autonomy to organize in the way they see fit.

Until George Floyd's death, Abdullah said BLMLA had only raised $100,000 in the last seven years, money that it used for "the basic cost of organizing," things like renting rooms, making copies, printing banners and hiring unarmed security. They also try to provide limited support to families who lose members to police brutality, like aiding with funeral costs, which Abdullah said can sometimes run up to $20,000.

And unless money is earmarked specifically for the local chapters, money donated through the main network goes to the larger global network, Abdullah said. Chapters can ask for a little money from the national organization, but it's not always a significant amount, she said.

But with donations streaming in after Floyd's death, BLMLA has more money than it has ever had.

"We're stepping back to take a minute and process what we should be doing with the newfound money that we've never had before," she said.

'All Lives Matter' and other backlash

With more notoriety comes more scrutiny, and the Black Lives Matter movement has experienced its fair share of criticism.

Many have condemned the more progressive demands of the cause, most notably the call to defund the police, which would instead invest those funds in communities, especially marginalized ones where much of the policing occurs.

Others have called the movement too radical.

Still, Michael Kazin, who studies social movements at Georgetown University, said it's surprising that the movement hasn't received more backlash. He said it's the first time in history that a movement focused on Black equality has seen this kind of support from White people.

"Even in the '60s, with the (March on Washington), polls at the time never had more than 15, 20% support among Whites," Kazin, who is also the co-editor of the democratic-socialist publication Dissent Magazine, told CNN.

"We're seeing huge crowds in little cities, or even where there's hardly any Black people," Kazin said. "That's new. That didn't happen in the '60s, or even in 2013."

Why people are rallying behind BLM

Even large corporations and brands, which may have been silent in the past, are getting involved.

With the rise of social media in the 2010s, images showing police brutality against Black people have reached a much wider audience. Now a whole generation has grown up exposed to the issue of systemic racism.

Or, as Minneapolis community organizer Shanene Herbert put it, the trauma.

More people are demanding change. And BLM has been a big motivator in that shift.

Mary Westbrook, who lives in Philadelphia and participated in the city's protests in June, told CNN that she felt it was important for her to support the movement.

"I've lived in this city for 16 years," the 38-year-old told CNN in a Twitter message in June. "I am a children's librarian at the library right there on the Parkway (which is why I know the area and streets there really well). I see every day what black and other POC are dealing with in this city."

In the last seven years, BLM activists have made their message mainstream, according to Marcia Chatelain, author of "Franchise: The Golden Arches in Black America."

"There's been a shift in the public and a willingness to engage with their ideas," Chatelain, an American studies professor at Georgetown University, told CNN.

For example, she said, "people are willing to take a second to understand what defunding the police and prison abolition are all about."

What the future holds

When asked what she hopes for BLM's future, co-founder Cullors said she knows the movement will win.

"I know this because our work is full of love, healing and dignity," she said. "And we center Black people's humanity and life over our death and decimation."

The task isn't an easy one. Kazin, the Georgetown professor, pointed out that the BLM movement is about power -- having power to decide how policing will look, or having power to access quality education.

BLM signs in windows, anti-racist reading lists -- all these things are good, Kazin said. But unless they are coupled with a true shift in power and larger political solidarity efforts, little will change.

Still, people are listening now. And Scales, like Cullors, is optimistic.

"At this point, we don't stop," she said. "We're not able to relax. It's now the opportunity to push for the true transformation that we've been working for."

CNN's Christina Zdanowicz contributed to this report.

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How Black Lives Matter went from a hashtag to a global rallying cry - CNN