This morning, when I tried to start the car, my battery was low, none of the dash lights worked when I turned the ignition on, and it didn't crank. The indoor lights were very dim. Yesterday, it started right up fine, and it was only driven for 5 minutes, so it is probably not a charging problem. I did not leave any lights on accidentally either. How could this happen?

I would check the battery connections first however what you described is not unusual for a battery that's on its way out, when my battery was going bad I had just drove it home from work then I got in it to go to the store 20 minutes later and it wouldn't start. Put a new battery in and it started right up.

How long do batteries last? Do they last longer if the car hasn't been used much?

4-5 years if used regularly, letting them sit and not be used shortends the life of them.

A couple additional points.

This happened to a friend once. Turned out their trunk light switch failed permanent on. So they didn't leave any lights on, but the vehicle did.

Also happened to a different friend. They didn't lock their car at home, and someone did a car prowl in the night. To be sneaky they didn't shut the door very hard. Friend comes out in the morning to a dead battery. They didn't notice that the door wasn't latched and ran the battery down over night.

Disconnect battery from car, and put on a charger and see if she recovers.

I have batteries last over a decade many times.

Batterys live longest when stored fully charged, and have a short life if stored less than fully charged Note that a less than fully charged battery may still start your car.

A battery in a car that sits or is used for short trip only, lives a hard life and wont last long. This is cause short trips dont give the alternator enough time to replace the electric in the battery lost while starting the motor. Short trips at night are worse as headlights take plwer from alternator that would have other wise come to recharging the battery from the starting discharge.

Batteries self discharge, maybe 5 to 10% per month, more so in hot weather than cold, even when disconnected. When connected, it may full discharge to the point of not starting the car in a month or two given the parasitic drains found in many cars.
This is extremely damaging!

So if you store the car for more than a month or so, put a charger on it, trickle charger. or once a ,o th fully recharge it. If short drive, then you need to put a charger on it periodically, or drive it long time (headlamps off for fastest result)
For long term storage, specially on modern cars, disconnect battery unless you keep a charger on it, or periodically charge it fully. measure voltage to get an idea over time what the discharge rate is.

When buy new battery, check the date code, make sure battery date code is only one or at most two months old, else you risk getting a battery that sat on a shelf all that time self discharging, reducing its life. Bring a volt meter to store, and make sure it is not a discharged one you buy. Good stores periodically recharge their stock, other dont. I have seen 8 month old batteries for sale, run away! check the date code, else you make end up with a turkey.

I recharged the battery with a trickle charger, and it started fine for the past couple of days.