This theory is started by a reddit user! 

They theory is basically about Mew being the ancestor etc! The writer(I think) wrote this: 

"In statistics the population mean (the average) is given the Greek symbol Mu (μ), which is pronounced very similar to Mew. The definition of the average is the sum of all the values in the population divided by the number of values in the population. This means that when you include all possible values the average will be the number which shows the central tendency. Now, Mew is said to contain the DNA of all other Pokémon in its body, but by itself, it's not particularly strong. It could be possible that Mew actually represents the average Pokémon from which all other Pokémon are derived. Just like how the Mu contains all possible values, Mew contains DNA from all Pokemon."

This basically mean this: MU, a Greek letter is pronounced very similar to "Mew"! MU also turns out to means average! He says that the definition of average "is the sum of all the values in the population divided by the number of values in the population"! This means that you add all into one and divided all into how much you  have added up! Mew is said to be the ancestor of all pokemon as it has the DNA ( deoxyribonucleic acid) of every pokemon! This means it has every Pokemon's cells as DNA is basically a container for cells, human or pokemon! MU contains all energy, Mew contains all DNA of pokemon!

No wonder rocket scientists wanted to use Mew's cells to create Mewtwo!

Mew has the ability to learn any TM, HM or Move Tutor moves! This lead to many people creating theories about this!