Banks are among the most hated institutions on the planet.

Yet Bill and Melinda Gates believe banking -- specifically the mobile variety -- can be the source of great good in the near future, helping to lift millions of people out of poverty.

In its annual letter published on Thursday, the Bill and Melinda Gates Foundation identified mobile banking as one of four exciting breakthroughs that will help improve the lives of people in poor countries faster in the next 15 years than at any other time in history.

"Mobile banking will help the poor transform their lives," the billionaire couple wrote in the letter.

At first glance, mobile banking is a surprising choice for the Gates Foundation to throw its incredible heft behind. In the past, the philanthropy, which controls over $42 billion in assets, has focused challenges like access to condoms and turning human waste into drinking water.

But now the power couple is thinking about the 2.5 billion adults the World Bank estimates are without a bank account.

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Literally stuffing cash in a mattress: People in rich countries can drive or walk to the bank, use digital services like PayPal and write checks that can be deposited, sometimes even by taking a picture with a smartphone.

Those aren't really options if you don't have a bank account. People in poor countries often keep cash at home, literally stuffing it in the mattress. Or they invest it in an asset like lifestock that can lose value over time.

"If their savings are in the form of jewelry or livestock, for example, they can't very well chip off tiny pieces to cover routine daily expenses," Bill and Melinda Gates wrote.

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Mobile = more control: Sending money to friends or relatives can also be difficult, with people in poor countries needing to take off from work to deliver it by hand or rely on a courier.

Borrowing money without a bank account can be even more problematic, with some lenders charging exorbitant rates.

"Digital banking will give the poor more control over their assets," the couple wrote.

Many developing countries like Kenya are rapidly adopting mobile banking. Gates estimates that by 2030, about 2 billion people who don't have a bank account today will be storing money and making mobile payments.

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Mobile roadblocks: But to get to that level, mobile banking needs to overcome some big hurdles.

One obvious problem is the need to increase access to mobile phones, especially in countries like Bangladesh where women have fallen way behind men in this category.

The Gates Foundation also highlighted the need to convince regulators to revamp their laws in a way that encourages innovators to enter.

And while digital currency is great, poor countries need to develop enough physical locations for people to be able to convert this money into hard currency.

Interestingly, the focus by Gates and others like Visa (V) and MasterCard (MA) on mobile banking in poor nations could mean residents there enjoy services that Americans don't have access to yet.

"Entrepreneurs in developing countries are doing exciting work -- some of which will 'trickle up' to developed countries over time," Gates said.