This is the 5198th most frequent Russian word.
нигде
"Nowhere."
In this sentence, "нигде" is used to emphasize the absence of the keys in all possible places.
Я нигде не могу найти свои ключи.
I can't find my keys anywhere.
Here, "нигде" conveys the idea that there was no presence of people in any location inside the house.
Нигде в доме не было ни души.
There was not a soul anywhere in the house.
In this instance, "нигде" indicates that the person has not encountered the individual in any place or location.