As writers we are like those making maps, trying to draw a picture of our thoughts and being guides for others in the vast landscape of knowledge (thanks to @haideralmosawi for introducing this metaphor). It is our job to make sure our readers can safely journey through our stories.
It's hard to put your complete thoughts into words. There will always be left those uncharted territories you didn't even realize you didn't know. It might leave some empty spots on the map. But those are up to the reader to fill. In creative writing, this is a good thing. It's good to leave something up to the imagination.
But when writing something factual there shouldn't be any big gaps in your text. It should be as comprehensive as possible. There is no room for interpretation. When you have the concept in your head it might feel like you know it all. But when you write it out you notice those uncharted territories. The places where you need to venture deeper to make your map complete.
It's also intriguing to leave some areas unmapped. Encourage others to start their own exploration at the borders of the knowledge and expand that what is known from there on.