It's been dubbed "dressgate".
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And it involves four colours: black and blue, and white and gold.
A Tumblr user, under the name "swiked", posted a photo of a dress and asked her followers for help. But it was not fashion advice she was after.
"Guys please help me," she wrote.
"Is this dress white and gold, or blue and black? Me and my friends can't agree and we are freaking the f--- out."
The conundrum has since gone viral, confusing and dividing social media users far and wide.
Another user named "BradTheLadLong" who is claiming responsibility for the photo said the image was designed to test "what scientists already know".
"Scientists have proven that when there is a big event in your life that is having a negative effect on you, your sighting of colour may vary and that is why some people are seeing black and blue," he said.
"It is merely all down to emotion and when the person seeing black and blue checks back another day, they might see the dress in white and gold because their mood has changed from the state they were in when previously viewing the dress."
