Five years old this week, Apple’s iTunes has grown to an unrivalled size and has sold over four billion tracks. iTunes has a share of around 70% of all of the digital music sold the world over.

Speculation puts Apple, and iTunes’ share of all sales of music at somewhere around the 28% mark over the next five years. This is partly due to its dominance in the digital music market, and the falling sales of CDs in bricks and mortar stores.

Digital music is expected to account for 40% of all music sales by 2012 according to figures released by InStat, which means that if Apple holds onto the digital music share it currently has, it will be responsible for more than a quarter of all music sales come 2012.

Paul Resnikoff, the editor of Digital Music News said:

I’m very skeptical about whether iTunes can be unseated, because there’s not a lot of consumer pain there.

iTunes came about when Apple bought the little program SoundJam MP, in 2000. If only they knew how big it would become and how the whole of the music industry would be changed forever.