Complex sentence: Today, non-standard cosmologies are usually not considered by cosmologists, because the essentials of the big bang theory have been confirmed by a wide enough range of complementary and detailed observations, and no non-standard cosmologies have reproduced the range of successes of the big bang model.
Split into simpler sentences: Today, non-standard cosmologies are usually not considered by cosmologists, because they think the essentials of the big bang theory have been confirmed by a wide enough range of complementary and detailed observations. Most believe there is little chance of the theory being replaced in its essentials, and that non-standard cosmologies have not come close to reproducing the range of successes of the big bang model.