A cryptocurrency should never "go dark". One of the key selling points of cryptocurrencies is that their decentralized nature makes the network resilient, difficult to censor, and difficult to control. Stellar is currently one of the top 10 cryptocurrencies at a market cap of around $2 billion and its network went down for 2 hours on May 15th. The network outage was noted by Tim Swanson (@ofnumbers) on Twitter. According to him, the outage was caused by a critical mass of nodes failing, causing a cascading failure.
The centralized nature of Stellar is an inconvenient fact that is well understood by few. The network uses the same validator design as Ripple, another infamously centralized cryptocurrency. In fact, Stellar’s chief scientist confirmed last month that decentralization is a concern for Stellar, writing: "Stellar’s FBA configuration is highly centralized."
In April, 3 researchers from KAIST published paper titled "Is Stellar As Secure As You Think?" as a submission to IEEE S&B 2019. The paper concluded that Stellar was centralized and that the network will fail after deleting only two nodes.
Stellar has responded to these concerns and state that they have a plan and are continuously working to decentralized Stellar.