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This systematic review synthesizes current research on echo chambers and filter bubbles to highlight the reasons for the dissent in echo chamber research on the existence, antecedents, and effects of the phenomenon. The review of 112 studies reveals that the lack of consensus in echo chamber research is based on different conceptualizations and operationalizations of echo chambers. While studies that have conceptualized echo chambers with homophily and utilized data-driven computational social science (CSS) methods have confirmed the echo chamber hypothesis and polarization effects in social media, content exposure studies and surveys that have explored the full spectrum of media exposure have rejected it. Most of these studies have been conducted in the United States, and the review emphasizes the need for a more comprehensive understanding of how echo chambers work in systems with more than two parties and outside the Global North. To advance our understanding of this phenomenon, future research should prioritize conducting more cross-platform studies, considering algorithmic filtering changes through continuous auditing, and examining the causal direction of the association between polarization, fragmentation, and the establishment of online echo chambers. The review also provides the advantages and disadvantages of different operationalizations and makes recommendations for studies in the European Union (EU), which will become possible with the upcoming Digital Services Act (DSA). Overall, this systematic review contributes to the ongoing scholarly discussion on the existence, antecedents, and effects of echo chambers and filter bubbles.