At least to me there is a difference. If you use the definite article, you feel the objects are more of a collective (group) where each member has something important in common with the other members of the group, perhaps acting together as a whole. If you use the indefinite article, the common traits are not that relevant to you and you feel about the objects as individuals.
So, if we have the sentences on their own without any context, they mean the following:
Menschen suchen die Wahrheit in der Wissenschaft.
This implies: each one does it mostly by himself/herself. It's more of a lone struggle for everybody. There is not much emphasis on collaboration.
Die Menschen suchen die Wahrheit in der Wissenschaft.
Probably, there is a lot of collaboration to achieve this goal.
Wölfe streifen im Wald umher.
Probably each one on its own. No emphasis on group behavior.
Die Wölfe streifen im Wald umher.
Probably forming a pack, collaborating on finding a victim.
Bäume stehen an der Straße.
Probably forming a tree-lined road. They have nothing special in common, other than their location along the road.
Die Bäume stehen an der Straße.
Probably forming a small forest. Or if not, perhaps a group of trees is being talked about each of which has to be cut.
Lehrer sind blöd.
Means if the speaker imagines a (single) teacher, he/she considers him stupid.
Die Lehrer sind blöd.
The teachers that the speaker cares about have something in common: probably they're all at the same school. And not each one of them has to be stupid (for this sentence to be true to the speaker) but they as a collective have to be stupid. He/she simply does not see them as individuals.
Kekse schmecken lecker.
Means if the speaker sees a (single) cookie, he/she considers it tasty.
Die Kekse schmecken lecker.
Probably only the cookies in a certain box or in a certain serving.
Person A: "Wo sind die Bonbons?" - Person B: "Da sind Bonbons." / "Da sind welche."
This answer implies that B is unsure if he/she has found the ones A was looking for.
Person A: "Wo sind die Bonbons?" - Person B: "Da sind die Bonbons." / "Da sind sie."
B thinks they are the ones that A was looking for.