かろう

Attached to the form where [い] of an い-adjective is omitted or to the negative form of [だ] as [ではない], to express speculation, nearly synonymous with [だろう].

The speculative form [V-よう] is the corresponding verb form for this usage.

This is an old-fashioned, literary expression often used in writing or formal speech. In everyday spoken language, [だろう] is used.