The phrase "existed side-by-side" can be replaced with various synonyms to add variety and depth to your writing. Words like coexisted, intermingled, intertwined, cohabited, and comingle are some of the alternatives you can use to convey the same meaning. The phrase can also be expressed as "parallel existence" or "simultaneous presence." Furthermore, "coincided," "concurrent," and "collaborated" are other suitable vocabulary choices. When writing, it's essential to vary your language to prevent repetitive phrases and add interest to your content. Using synonyms creatively and effectively will improve your writing's impact and make it more enjoyable to read.