You may have noticed a lot of people posting mysterious black and white photos with the words 'challenge accepted' to Facebook recently.

It's a new social media craze filling up our newsfeeds - but it's left many of us scratching our heads.

It basically involves people posting black and white photos of themselves to Facebook and Instagram with the caption 'challenge accepted', often tagging a friend in the comments to challenge them to do the same.

The problem is, there doesn't seem to be a solid explanation as to its origins.

A lot of people are saying it's to raise awareness and show support for the battle against cancer.

But according to the Mirror , it doesn't appear to be affiliated with any major charity, and donations or any sort of action aren't being encouraged either.

The Facebook craze, which uses the hashtag #challengeaccepted, has seen some users posting their black and white images with no explanation at all.

The trend has been spreading around the world and was incredibly popular in India at the beginning of August, finally catching on in the UK in the last week.