There isn't a number that you can fill in that blank and make it equal zero.
The bigger and bigger the number you put, the closer it gets to zero, but no number that you put in there will make it equal zero. You can always think of a bigger number and put that in there, and that will make it closer to zero. There is always going to be a number that is one bigger than the last number you tried.
25 divided by 1,000 is 0.025
25 divided by one billion is 0.000000025
25 divided by fifty quadrillion is 0.0000000000000005
And, as you can imagine, 25 divided by a super googol is going to be some huge long decimal that is almost zero, but not quite.
And a super googol and one will get even closer to zero.
And so on....
You can kind of say that the answer is infinity, but infinity is not a number.