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By Devin Coldewey

Twitter has collected the trends and viral hits of 2015 — both the happy moments and the sad ones. It was a big year for social change, landmark political decisions — and One Direction. Here are some highlights from the social network's lists of the top hashtags and tweets.

  • #PrayForParis and #JeSuisCharlie amassed millions of tweets from enemies of terrorism worldwide after both of the deadly tragedies in the French capital.
  • #BlackLivesMatter helped unify separate discussions decrying racism and police brutality.
  • Marriage equality legislation was celebrated in the U.S. with #LoveWins and in Ireland with #HomeToVote. President Barack Obama's tweet supporting the Supreme Court's ruling declaring same-sex marriage legal across all states was also retweeted nearly half a million times.
  • #RefugeesWelcome showed people fleeing conflicts and genocide that they had a home waiting in Europe.
  • After 14-year-old Ahmed Mohammed was cuffed when teachers thought his homemade clock was a bomb, 370,000 people tweeted to say #IStandWithAhmed.
  • One Direction's members each produced hundreds of thousands of retweets and favorites as they celebrated five years together and parted ways with member Zayn Malik.
  • The Women's World Cup attracted plenty of attention, with related tweets garnering 9 billion views.
  • Millions of miles away, the New Horizons probe capped a 9-year journey with its successful #PlutoFlyBy.
  • Caitlyn Jenner collected a million followers in just four hours, breaking a record set by President Obama.
  • And who could forget the colorful debate over #TheDress?

The rest of the year's most memorable or retweetable moments can be found at Twitter's 2015 trends page.