This poster presents the NFDI4Memory Data Space App. Historical data are extremely heterogeneous and often tied to individual projects, which makes reuse very difficult. The Data Space App addresses this challenge by integrating archives, repositories, and services into a unified digital ecosystem. At the core of the architecture there are standardized APIs together with the MemO Ontology. These components enable semantic alignment and machine-actionable linking across institutions. The Data Space App provides a user-friendly interface. Built with FastAPI and Fuseki/Jena on the backend and Vue.js on the frontend, it delivers unified search, metadata harmonization, enrichment pipelines, and modular services. The infrastructure will expand through additional APIs, new repository integrations, and deeper semantic interconnections. Ultimately, the Data Space App aims to become a sustainable backbone for FAIR and interoperable historical research.