隙

without any weak spot

niche market

spare moment

gap

peeping

to wait for an opportune moment

void ratio

before one could

niche industry

niche product

God of the gaps

gap

synaptic cleft

closely

draft

vacant space

one must constantly be on one's guard with ...

slit

potential space

gap

to watch for an unguarded moment

chink in the armor (armour)

to exploit a gap (in attention, security, etc.)

crevice

porosity

time flies

spare time

caulking

gap

crack