What is this unusual structure inside this banana?
It looks like some species of Nigrospora genus took over the inside of that banana. According to Wikipedia, N. sphaerica was first isolated from bananas. The other species that you will find referenced online is N. oryzae. In the articles I 
cite below the colonies are grown in potato dextrose, though.

Here is a picture of the red fungus inside a banana (from the Canadian Food Inspection Agency): 



Googling 'Nigrosporia banana' also returns lots of similar pictures.

To get a better answer, a sample could be taken from the banana and then cross-check what you see under the microscope with the oryzae morphology images seen in the researchgate article.