Ventured guess is a phrase that refers to an uncertain prediction or estimation. Antonyms for venturing a guess could include certainty, truth, knowledge, or fact. When we venture a guess, we take a risk by making a conjecture without full information. The opposite of this would be to base a conclusion on facts and reasoning. Words such as determined, convinced, established, and proven are the antonyms for venturing a guess. It's important to note that venturing a guess is not necessarily a negative thing, as it can be used to explore or test ideas. However, when making important decisions, we should strive to base them on accurate information and logical reasoning.