The official EU service that says this VAT number is real, today.
VIES (VAT Information Exchange System) is the European Commission's public service for checking whether a VAT number is valid for intra-EU transactions. A request goes out to the issuing member state's own database in real time and comes back with a yes or no, the trader name and address where that member state shares them, and a consultation number that proves the check happened.
The service is reachable from the public Vies-on-the-Web page, the SOAP checkVatService endpoint that ERPs and accounting tools have used for years, and a more recent REST endpoint. It covers the 27 EU member states. The European Commission is explicit that VIES does not store the data itself and is not responsible for accuracy: the answer is only as fresh as the national tax authority's own database. That is exactly why the consultation number matters. It is the audit-grade proof that on a given date, that VAT number was valid according to the home country's records.