Okay so perhaps this doesn't fit into the timescale but I'm taking Dark Ages as a relative term.
How and why eastern Bengal became so thoroughly Islamised. This is quite a puzzling situation and nobody has really come up with a definitive answer. We don't even know when this area of Bengal became Muslim - was it during the Mughal era? Was it before? Who were the key figures in this transition and why did it happen this way in Bengal and nowhere else in the subcontinent? Eastern Bengal was far away from the centres of Islamic power in the subcontinent, and many Muslim rulers saw it as a distant realm where one could exile political opponents and not have to worry about them.
It wasn't until the British conducted ethnographical surveys in the late 19th century that scale of Islamisation in Bengal was apparent. Nobody - not even the Indians - realised that eastern Bengali was majority Muslim! The British fully expected the region to be majority Hindu (and less populated than it actually was). Numerous theories were proposed to explain this but a lot is complete conjecture. A popular theory within Bengal is that low-caste Hindus converted en masse to Islam to escape caste prejudices and the upper castes followed suit thereafter (a bottom-up approach). This hypothesis has many holes however, as elucidated by Richard Eaton. One of the most interesting points he raises is that eastern Bengal was not really "Hindu" in a meaningful sense of the word, but rather outwith the fold of Vedic civilisation and populated mostly by villagers who followed a folk religion influenced but not penetrated by Dharmic currents. This relates to Bronkhorst's vision of a "Greater Magadhan" cultural sphere, centred around the Bengal delta region, which existed in opposition to the Vedic Brahmin culture of northern India. This Magadhan culture is conjectured to be what gave rise to Buddhism and Jainism, as sramanic traditions that didn't grow out of Brahminism but grew separately from indigenous philosophical and cultural ideas. Eastern Bengal may have been the last vestige of the "Magadhan" civilisation and was actually Islamised before it was ever Hindu-ised.
Again though, we just don't know what really happened.