Evidence

Throughout the novel, the narrators tell us they've been collecting "evidence' about the Lisbon sisters, both before and after the mass suicides. They steal stuff from the girls' bedrooms, go through the trash after their deaths, and order catalogs they pilfer from the Lisbon mailbox, desperately trying to understand who these girls are and what's happened to them. The "evidence" consists of physical stuff that belonged to the girls and photos and documents they've acquired. Some of it's pretty intimate: hairbrushes with the girls' hair on them, for instance, and medical records from their doctor visits. The boys present this "evidence" to the reader as if they're all being really objective about it. Later in life, the narrators interview doctors the girls saw and track down Mr. and Mrs. Lisbon to get more info.

The items they collect over the years are obsessively catalogued and kept by the boys even as middle-aged men:

All of it is going—Exhibits #1 through #97, arranged in five separate suitcases, each bearing a photograph of the deceased like a Coptic headstone, and kept in our refurbished treehouse in one of our last trees: (#1) Ms. D'Angelo's Polaroid of the house, scummed by a greenish patina that looks like moss; (#18) Mary's old cosmetics drying out and turning to beige dust; (#32) Cecilia's canvas high-tops yellowing beyond remedy of toothbrush and dish soap; (#57) Bonnie's votive candles nibbled nightly by mice; (#62) Therese's specimen slides showing new invading bacteria; (#81) Lux's brassiere […] We haven't kept our tomb sufficiently airtight, and our sacred objects are perishing. (5.39)

But the narrators realize the futility of it all:

In the end, we had pieces of the puzzle, but no matter how we put them together, gaps remained, oddly shaped emptinesses mapped by what surrounded them, like countries we couldn't name. […] So much has been written about the girls in the newspapers, so much has been said over backyard fences, or related over the years in psychiatrists' offices, that we are certain only of the insufficiency of explanations. (5.40)

This is exactly what the "evidence" represents—the inability of anyone to really know what's going on in the mind and heart of another person. The boys' perspective on the Lisbon girls has been a distant one, the perspective of observers, not participants. Same goes for the neighbors, who have watched the tragedy unfold from a distance. In a sense, the family wasn't more than an object of curiosity for many people. But the boys learned all too well that the outward trappings of a life tell you very little about the person within.