Under art.11 Regulation 1/2003, every national competition authority (NCA) of the EU Member States must notify the EU Commission about the opening of formal investigations for a potential infringement of arts 101-102 TFEU and the envisaged decisions in that regard. The enforcement statistics for the first decade of Regulation 1/2003 indicate that the number of notified envisaged decisions is only about 50 per cent of the number of investigations opened under EU competition law. The authors explore the resulting "dark matter" in EU competition law by looking specifically at the non-infringement and closure decisions adopted by NCAs under national competition rules. The article assesses the compatibility of such decisions with the powers of NCAs under art.5 Regulation 1/2003, as interpreted in the EC Ts Tele2 Polska case law. The article shows how the ECJ's interpretation of art.5 Regulation 1/2003 has contributed to the development of the "dark matter" in the decentralised system of EU competition law enforcement, which affects the ability of the EU Commission to monitor the enforcement activities of the NCAs.