There is a related question which I read, under which someone answered that the 的 makes it feel more natural. Is there a grammatical reason for the character, or is it a matter of emphasis?
Personally, not scientifically, I believe this usage stems from the Chinese penchant for wrapped phrases. There are many such collocations.
If you ask, people will say, this 的 is just a tone help.
Now [something] is wrapped betwixt 我不知道 and 的 and this is pleasing to the Chinese ear.
Where I live people often ask: 啊懂啊？，懂 neatly wrapped in two 啊