Paris attacks, #BlackLivesMatter, One Direction and more top Twitter trends of 2015

Twitter is looking back on 2015 with the hashtag #YearOnTwitter. This year saw more people than ever before people tweet, retweet, and "like" -- now, complete with a heart icon -- posts on a wide range of stories sweeping through the news. Here's a look at the most popular hashtags connecting us to stories that grabbed the world's attention over the past year.

#ParisAttacks

In the wake of the Nov. 13 Paris terror attacks, Twitter users came together to mourn and commemorate the victims as well as show support for the French capital. Many also expressed their sorrow with the hashtag #PrayforParis.

#RefugeesWelcome

2015 saw hundreds of thousands of refugees and migrants flee parts of the Middle East and Africa to Europe. Many lost their lives in the process. While the influx of desperate people prompted a backlash against them, many activists, human rights watchers, and people from all over the world took to Twitter to express their support for the refugees.

#BlackLivesMatter

The Black Lives Matter movement became a dominant trending topic in 2015 as protests against inequality and police brutality spread to cities across the U.S. The hashtag was tweeted 9 million times over the past year.

#MarriageEquality

Following the Supreme Court's ruling in June 2015 that legalized same-sex marriage nationwide, talk of marriage equality dominated Twitter. Many celebrated with the hashtag #LoveWins.

#TheDress

Few things sparked as much debate in 2015 as the infamous viral dress photo. A picture of a dress that appeared to be different colors to different people -- black and white to some, blue and gold to others -- made the optical illusion a globally trending topic. From Feb. 26 to 27, Twitter says the image was included in more than 4.4 million tweets.

#IStandWithAhmed

The story of 14-year-old Ahmed Mohammed generated outrage across social media, when the student was put in handcuffs at his Texas school for bringing in a homemade digital clock to school that some mistook for a bomb. More than 370,000 tweets were generated globally when the image of the student in handcuffs went viral. Even President Obama go involved, tweeting: "Cool clock, Ahmed. Want to bring it to the White House? We should inspire more kids like you to like science. It's what makes America great."

#FIFAWWC

The FIFA Women's World Cup in Canada became a global sporting sensation this summer, with tweets during the competition from June 6 to July 5 being viewed 9 billion times.

#PlutoFlyby

Twitter users generated more than 1 million tweets when NASA's New Horizons spacecraft flew within 7,770 miles of Pluto on July 14.

@Caitlyn_Jenner

The world met a new social media star in June, when Caitlyn Jenner joined Twitter. The Olympian-turned-reality TV star gained 1 million Twitter followers in a little over four hours from the moment @Caitlyn_Jenner hit the scene, earning a spot in the Guinness Book of World Records.

The "Golden Tweet"

Beyond top trending topics, what was this year's "Golden Tweet?" Twitter announced that 2015's most re-tweeted tweet was from One Direction star Harry Styles. It came in response to the news that band member Zayn Malik was leaving the group. As tween hearts everywhere were breaking, Styles calmly reassured them by tweeting: "All the love as always."