An observation that indicates in what sense idealism is right in claiming that the world has an ideal (unreal? not in every sense of the word) character: that we feel that when people are not part of the world (not yet, or no longer, i.e., before the human species developed, or after its demise) it has a completely different character.
For some the realisation that there comes a time when all traces of human existence have disappeared is a consolation. (A consolation that is temporally mirrored in the attractiveness of a state of the world that is not yet touched by humans.) Why is this a consoling thought? If we try to imagine the world without humans –which is extremely difficult, if not paradoxical, since there always is this imagining subject (cf., on this Schopenhauer, in the chapter on death in the second part of Die Welt als Wille und Vorstellung)– then the result is an image that is quiet, silent. Of course, the normal physical (geological, biological) processes at different scales (from cosmos to microcosmos) will continue. In that sense, nothing changes, nothing is different. What nevertheless creates silence, a kind of peace even, is that there are no humans as sources of desires, thoughts, emotions, consciousness. The world without humans can be said to be, in its own right, blind, without consciousness, without past or future. For, as far as we humans are concerned, these are characteristics of the world that we create, that spring forth from us as conscious beings.
And for the very same reasons that the thought of a world with human traces is a consolation for some, it is abhorrent and frightening for others. Those who are tired of the consciousness that characterises humans, who want to be rid of it, will find comfort in this thought. But for others it is a source of anxiety since it does away with what they hold dear most: the identity of the individual consciousness.
In what sense does this show that idealism is true? It is precisely because the consolation and anxiety it produces shows how fundamentally different a world with humans is from one without. It is in that sense that human consciousness creates the world in which it manifests itself. That world is drenched in it, filled with it to the brim, in both material and immaterial ways. All forms of activity, intentional actions, the concepts of goals and means, of significance and insignificance, of past and future, are reflections of that consciousness and the desires and cravings that are its source. In that sense consciousness creates the world, while at the same time as a causal-material phenomenon it is created by that world. And it is in that sense that a world without consciousness is a un-world, something that is not.