A prevailing concern in higher education, especially within the fields of Science, Technology, Engineering, and Mathematics (STEM), often refers to underrepresentation of certain social groups. While the public attention is drawn to the aggregated statistics reflecting participation issues at a macro societal level, the essence of the debate is the real underrepresented individuals behind each number that have been hindered by any tangible or impalpable reasons. Thus, how to address existing disparities in education systems by looking beyond the aggregate number to the nuanced barriers faced by different groups becomes vital. Based on a systematic literature review, this research aims to explore the complex barriers and opportunities regarding the expansion of participation in STEM education. By analyzing global examples, the study identifies socioeconomic disadvantages, exclusionary institutional practices, and internal psychological barriers such as low confidence, lack of belonging, and stereotype threat as key factors that hinder the engagement of marginalized communities in STEM fields. Drawn on a meta-analysis, the research also evaluates successful intervention practices, such as role model-based initiatives and inclusive educational environments, and values-affirmation exercises, which have been shown to improve motivation, strengthen self-efficacy, and increase retention in STEM. Additionally, the study synthesizes these findings to propose systemic reforms at individual, institutional, and governmental levels. In summary, this research offers evidence-based and actionable strategies, leading to a more inclusive STEM landscape for sustainable development.