Agar.io is an online multiplayer game where players control a cell in a petri dish. The goal is to grow by eating colored dots and other smaller cells, while avoiding larger ones. It offers modes such as FFA, team, experimental, and party. It includes mechanics such as splitting to attack or escape, and strategically using viruses. It was created in 2015 by Matheus Valadares and quickly gained popularity thanks to its simple yet addictive gameplay, going viral on social media and platforms like YouTube and Twitch.