The fat earth theory is a satirical theory proposed by Lars.
The theory exists in contrast to the real-world flat earth theory, which is a hypothesis that the earth is not a spheroid, but rather a flat disc of some sort. Lars jokingly started a meme about Tiege being a flat earth conspiracy theorist.
According to Lars, the earth is currently in danger of "falling out of the sky". He hypothesizes that this is due to the international increase in obesity in recent years.